Add build_script_env_files crate annotation (#4166)

## Summary

Add `build_script_env_files` support to `crate.annotation`, allowing
environment files to be passed to generated `cargo_build_script`
targets.

The change:

- Exposes `build_script_env_files` through bzlmod and legacy crate
annotations.
- Propagates the annotation through crate-universe configuration and
build-script context.
- Renders the attribute on generated `cargo_build_script` rules.
- Supports unconditional and platform-selected values.
- Updates the configuration digest.
- Adds tests for annotation propagation and BUILD-file rendering.

## Testing

- `bazel test //crate_universe:unit_test`