Add UV_FIND_LINKS environment variable support for the --find-links command-line option (#7912)

## Summary

This PR adds support for the `UV_FIND_LINKS` environment variable as an
alternative to the `--find-links` command-line option, as requested in
#1839.

## Test Plan

A unit test was added to validate that setting `UV_FIND_LINKS` provided
the same result as a link provided with the `--find-links` command-line
option.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

docs/reference/cli.md

@@ -148,6 +148,7 @@ uv run [OPTIONS] <COMMAND>
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--frozen</code></dt><dd><p>Run without updating the <code>uv.lock</code> file.</p>
 
 <p>Instead of checking if the lockfile is up-to-date, uses the versions in the lockfile as the source of truth. If the lockfile is missing, uv will exit with an error. If the <code>pyproject.toml</code> includes changes to dependencies that have not been included in the lockfile yet, they will not be present in the environment.</p>
@@ -692,6 +693,7 @@ uv add [OPTIONS] <PACKAGES|--requirements <REQUIREMENTS>>
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--frozen</code></dt><dd><p>Add dependencies without re-locking the project.</p>
 
 <p>The project environment will not be synced.</p>
@@ -1009,6 +1011,7 @@ uv remove [OPTIONS] <PACKAGES>...
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--frozen</code></dt><dd><p>Remove dependencies without re-locking the project.</p>
 
 <p>The project environment will not be synced.</p>
@@ -1314,6 +1317,7 @@ uv sync [OPTIONS]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--frozen</code></dt><dd><p>Sync without updating the <code>uv.lock</code> file.</p>
 
 <p>Instead of checking if the lockfile is up-to-date, uses the versions in the lockfile as the source of truth. If the lockfile is missing, uv will exit with an error. If the <code>pyproject.toml</code> includes changes to dependencies that have not been included in the lockfile yet, they will not be present in the environment.</p>
@@ -1617,6 +1621,7 @@ uv lock [OPTIONS]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--frozen</code></dt><dd><p>Assert that a <code>uv.lock</code> exists, without updating it</p>
 
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
@@ -1896,6 +1901,7 @@ uv export [OPTIONS]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--format</code> <i>format</i></dt><dd><p>The format to which <code>uv.lock</code> should be exported.</p>
 
 <p>At present, only <code>requirements-txt</code> is supported.</p>
@@ -2204,6 +2210,7 @@ uv tree [OPTIONS]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--frozen</code></dt><dd><p>Display the requirements without locking the project.</p>
 
 <p>If the lockfile is missing, uv will exit with an error.</p>
@@ -2575,6 +2582,7 @@ uv tool run [OPTIONS] [COMMAND]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--from</code> <i>from</i></dt><dd><p>Use the given package to provide the command.</p>
 
 <p>By default, the package name is assumed to match the command name.</p>
@@ -2863,6 +2871,7 @@ uv tool install [OPTIONS] <PACKAGE>
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--force</code></dt><dd><p>Force installation of the tool.</p>
 
 <p>Will replace any existing entry points with the same name in the executable directory.</p>
@@ -3147,6 +3156,7 @@ uv tool upgrade [OPTIONS] <NAME>...
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
 </dd><dt><code>--index-strategy</code> <i>index-strategy</i></dt><dd><p>The strategy to use when resolving against multiple index URLs.</p>
@@ -4711,6 +4721,7 @@ uv pip compile [OPTIONS] <SRC_FILE>...
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--generate-hashes</code></dt><dd><p>Include distribution hashes in the output file</p>
 
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
@@ -5099,6 +5110,7 @@ uv pip sync [OPTIONS] <SRC_FILE>...
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
 </dd><dt><code>--index-strategy</code> <i>index-strategy</i></dt><dd><p>The strategy to use when resolving against multiple index URLs.</p>
@@ -5441,6 +5453,7 @@ uv pip install [OPTIONS] <PACKAGE|--requirement <REQUIREMENT>|--editable <EDITAB
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
 </dd><dt><code>--index-strategy</code> <i>index-strategy</i></dt><dd><p>The strategy to use when resolving against multiple index URLs.</p>
@@ -6610,6 +6623,7 @@ uv venv [OPTIONS] [PATH]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
 </dd><dt><code>--index-strategy</code> <i>index-strategy</i></dt><dd><p>The strategy to use when resolving against multiple index URLs.</p>
@@ -6857,6 +6871,7 @@ uv build [OPTIONS] [SRC]
 
 <p>If a URL, the page must contain a flat list of links to package files adhering to the formats described above.</p>
 
+<p>May also be set with the <code>UV_FIND_LINKS</code> environment variable.</p>
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
 </dd><dt><code>--index-strategy</code> <i>index-strategy</i></dt><dd><p>The strategy to use when resolving against multiple index URLs.</p>