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/lib.rs

@@ -21,7 +21,7 @@ pub use crate::interpreter::{
 };
 pub use crate::pointer_size::PointerSize;
 pub use crate::prefix::Prefix;
-pub use crate::python_version::PythonVersion;
+pub use crate::python_version::{BuildVersionError, PythonVersion};
 pub use crate::target::Target;
 pub use crate::version_files::{
     DiscoveryOptions as VersionFileDiscoveryOptions, FilePreference as VersionFilePreference,