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/installation.rs

@@ -252,6 +252,7 @@ impl PythonInstallation {
         installed.ensure_externally_managed()?;
         installed.ensure_sysconfig_patched()?;
         installed.ensure_canonical_executables()?;
+        installed.ensure_build_file()?;
 
         let minor_version = installed.minor_version_key();
         let highest_patch = installations