Add install_mirrors to EnvironmentOptions (#15937)

## Summary

Add install_mirrors to EnvironmentOptions
Relates #14720

## Test Plan

Tests with existing tests

crates/uv-settings/src/lib.rs

@@ -570,6 +570,7 @@ pub enum Error {
 pub struct EnvironmentOptions {
     pub python_install_bin: Option<bool>,
     pub python_install_registry: Option<bool>,
+    pub install_mirrors: PythonInstallMirrors,
 }
 
 impl EnvironmentOptions {
@@ -580,6 +581,17 @@ impl EnvironmentOptions {
             python_install_registry: parse_boolish_environment_variable(
                 EnvVars::UV_PYTHON_INSTALL_REGISTRY,
             )?,
+            install_mirrors: PythonInstallMirrors {
+                python_install_mirror: parse_string_environment_variable(
+                    EnvVars::UV_PYTHON_INSTALL_MIRROR,
+                )?,
+                pypy_install_mirror: parse_string_environment_variable(
+                    EnvVars::UV_PYPY_INSTALL_MIRROR,
+                )?,
+                python_downloads_json_url: parse_string_environment_variable(
+                    EnvVars::UV_PYTHON_DOWNLOADS_JSON_URL,
+                )?,
+            },
         })
     }
 }
@@ -635,3 +647,24 @@ fn parse_boolish_environment_variable(name: &'static str) -> Result<Option<bool>
 
     Ok(Some(value))
 }
+
+/// Parse a string environment variable.
+fn parse_string_environment_variable(name: &'static str) -> Result<Option<String>, Error> {
+    match std::env::var(name) {
+        Ok(v) => {
+            if v.is_empty() {
+                Ok(None)
+            } else {
+                Ok(Some(v))
+            }
+        }
+        Err(e) => match e {
+            std::env::VarError::NotPresent => Ok(None),
+            std::env::VarError::NotUnicode(err) => Err(Error::InvalidEnvironmentVariable {
+                name: name.to_string(),
+                value: err.to_string_lossy().to_string(),
+                err: "expected a valid UTF-8 string".to_string(),
+            }),
+        },
+    }
+}

crates/uv-settings/src/settings.rs

@@ -22,7 +22,6 @@ use uv_resolver::{
     AnnotationStyle, ExcludeNewer, ExcludeNewerPackage, ExcludeNewerTimestamp, ForkStrategy,
     PrereleaseMode, ResolutionMode,
 };
-use uv_static::EnvVars;
 use uv_torch::TorchMode;
 use uv_workspace::pyproject::ExtraBuildDependencies;
 use uv_workspace::pyproject_mut::AddBoundsKind;
@@ -958,7 +957,7 @@ pub struct ResolverInstallerSchema {
 }
 
 /// Shared settings, relevant to all operations that might create managed python installations.
-#[derive(Debug, Clone, PartialEq, Eq, Deserialize, CombineOptions, OptionsMetadata)]
+#[derive(Debug, Clone, Default, PartialEq, Eq, Deserialize, CombineOptions, OptionsMetadata)]
 #[serde(rename_all = "kebab-case")]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct PythonInstallMirrors {
@@ -1007,26 +1006,15 @@ pub struct PythonInstallMirrors {
     pub python_downloads_json_url: Option<String>,
 }
 
-impl Default for PythonInstallMirrors {
-    fn default() -> Self {
-        Self::resolve(None, None, None)
-    }
-}
-
 impl PythonInstallMirrors {
-    pub fn resolve(
-        python_mirror: Option<String>,
-        pypy_mirror: Option<String>,
-        python_downloads_json_url: Option<String>,
-    ) -> Self {
-        let python_mirror_env = std::env::var(EnvVars::UV_PYTHON_INSTALL_MIRROR).ok();
-        let pypy_mirror_env = std::env::var(EnvVars::UV_PYPY_INSTALL_MIRROR).ok();
-        let python_downloads_json_url_env =
-            std::env::var(EnvVars::UV_PYTHON_DOWNLOADS_JSON_URL).ok();
+    #[must_use]
+    pub fn combine(self, other: Self) -> Self {
         Self {
-            python_install_mirror: python_mirror_env.or(python_mirror),
-            pypy_install_mirror: pypy_mirror_env.or(pypy_mirror),
-            python_downloads_json_url: python_downloads_json_url_env.or(python_downloads_json_url),
+            python_install_mirror: self.python_install_mirror.or(other.python_install_mirror),
+            pypy_install_mirror: self.pypy_install_mirror.or(other.pypy_install_mirror),
+            python_downloads_json_url: self
+                .python_downloads_json_url
+                .or(other.python_downloads_json_url),
         }
     }
 }
@@ -2270,11 +2258,11 @@ impl From<OptionsWire> for Options {
             build_constraint_dependencies,
             environments,
             required_environments,
-            install_mirrors: PythonInstallMirrors::resolve(
+            install_mirrors: PythonInstallMirrors {
                 python_install_mirror,
                 pypy_install_mirror,
                 python_downloads_json_url,
-            ),
+            },
             conflicts,
             publish: PublishOptions {
                 publish_url,