Support using uv build-backend in the Python backend (#13049)

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## Summary

Provide an in-code switch to permit using the `uv build-backend` command
rather than the default `uv-build` in the Python PEP517 backend. This
option is intended to be used by downstream packagers to provide an
option of reusing `uv` that was built already instead of having to build
a second `uv-build` executable that largely overlaps with `uv`.

Fixes #12389

## Test Plan

The option is intended for downstream consumption only, and it is tested
downstream (via attempting to build a package using the `uv_build`
backend). The backend itself is covered by tests already.

---------

Co-authored-by: konstin <konstin@mailbox.org>

crates/uv-build/python/uv_build/__init__.py

@@ -21,6 +21,11 @@ if TYPE_CHECKING:
     from collections.abc import Mapping, Sequence  # noqa:I001
     from typing import Any  # noqa:I001
 
+# Use the `uv build-backend` command rather than `uv-build`. This options is provided
+# for downstream distributions who provide `uv` and wish to avoid building a partially
+# overlapping `uv-build` executable.
+USE_UV_EXECUTABLE = False
+
 
 def warn_config_settings(config_settings: "Mapping[Any, Any] | None" = None) -> None:
     import sys
@@ -38,12 +43,17 @@ def call(
     import sys
 
     warn_config_settings(config_settings)
+
+    uv_bin_name = "uv" if USE_UV_EXECUTABLE else "uv-build"
     # Unlike `find_uv_bin`, this mechanism must work according to PEP 517
-    uv_bin = shutil.which("uv-build")
+    uv_bin = shutil.which(uv_bin_name)
     if uv_bin is None:
-        raise RuntimeError("uv was not properly installed")
+        raise RuntimeError(f"{uv_bin_name} was not properly installed")
+    build_backend_args = ["build-backend"] if USE_UV_EXECUTABLE else []
     # Forward stderr, capture stdout for the filename
-    result = subprocess.run([uv_bin, *args], stdout=subprocess.PIPE)
+    result = subprocess.run(
+        [uv_bin, *build_backend_args, *args], stdout=subprocess.PIPE
+    )
     if result.returncode != 0:
         sys.exit(result.returncode)
     # If there was extra stdout, forward it (there should not be extra stdout)
@@ -51,7 +61,10 @@ def call(
     sys.stdout.writelines(stdout[:-1])
     # Fail explicitly instead of an irrelevant stacktrace
     if not stdout:
-        print("uv subprocess did not return a filename on stdout", file=sys.stderr)
+        print(
+            f"{uv_bin_name} subprocess did not return a filename on stdout",
+            file=sys.stderr,
+        )
         sys.exit(1)
     return stdout[-1].strip()