Allow pinning managed Python versions to specific build versions (#15314)

Allows pinning the Python build version via environment variables, e.g.,
`UV_PYTHON_CPYTHON_BUILD=...`. Each variable is implementation specific,
because they use different versioning schemes.

Updates the Python download metadata to include a `build` string, so we
can filter downloads by the pin. Writes the build version to a file in
the managed install, e.g., `cpython-3.10.18-macos-aarch64-none/BUILD`,
so we can filter installed versions by the pin.

Some important follow-up here:

- Include the build version in not found errors (when pinned)
- Automatically use a remote list of Python downloads to satisfy build
versions not present in the latest embedded download metadata

Some less important follow-ups to consider:

- Allow using ranges for build version pins

crates/uv-python/src/managed.rs

@@ -320,6 +320,10 @@ pub struct ManagedPythonInstallation {
     ///
     /// Empty when self was constructed from a path.
     sha256: Option<Cow<'static, str>>,
+    /// The build version of the Python installation.
+    ///
+    /// Empty when self was constructed from a path without a BUILD file.
+    build: Option<Cow<'static, str>>,
 }
 
 impl ManagedPythonInstallation {
@@ -329,6 +333,7 @@ impl ManagedPythonInstallation {
             key: download.key().clone(),
             url: Some(download.url().clone()),
             sha256: download.sha256().cloned(),
+            build: download.build().map(Cow::Borrowed),
         }
     }
 
@@ -342,11 +347,19 @@ impl ManagedPythonInstallation {
 
         let path = std::path::absolute(&path).map_err(|err| Error::AbsolutePath(path, err))?;
 
+        // Try to read the BUILD file if it exists
+        let build = match fs::read_to_string(path.join("BUILD")) {
+            Ok(content) => Some(Cow::Owned(content.trim().to_string())),
+            Err(err) if err.kind() == io::ErrorKind::NotFound => None,
+            Err(err) => return Err(err.into()),
+        };
+
         Ok(Self {
             path,
             key,
             url: None,
             sha256: None,
+            build,
         })
     }
 
@@ -455,6 +468,11 @@ impl ManagedPythonInstallation {
         self.key.platform()
     }
 
+    /// The build version of this installation, if available.
+    pub fn build(&self) -> Option<&str> {
+        self.build.as_deref()
+    }
+
     pub fn minor_version_key(&self) -> &PythonInstallationMinorVersionKey {
         PythonInstallationMinorVersionKey::ref_cast(&self.key)
     }
@@ -618,6 +636,15 @@ impl ManagedPythonInstallation {
         Ok(())
     }
 
+    /// Ensure the build version is written to a BUILD file in the installation directory.
+    pub fn ensure_build_file(&self) -> Result<(), Error> {
+        if let Some(ref build) = self.build {
+            let build_file = self.path.join("BUILD");
+            fs::write(&build_file, build.as_ref())?;
+        }
+        Ok(())
+    }
+
     /// Returns `true` if the path is a link to this installation's binary, e.g., as created by
     /// [`create_bin_link`].
     pub fn is_bin_link(&self, path: &Path) -> bool {