cargo_build_script_runner: parse cargo::metadata key/value pairs (#3877)

## Summary
Fix `cargo::metadata=KEY=VALUE` handling in `cargo_build_script_runner`
so metadata keys are propagated in Cargo-compatible form.

Before:
- `cargo::metadata=version_1_10_0=1`
- produced `DEP_<links>_METADATA=version_1_10_0=1`

After:
- same input produces `DEP_<links>_VERSION_1_10_0=1`

## Changes
- Add a `"metadata"` parser branch in `BuildScriptOutput::new`.
- Split metadata payload on first `=` and emit
`DepEnv("<KEY>=<VALUE>")`.
- Preserve fallback behavior for malformed metadata payloads.
- Add unit test: `metadata_directive_maps_to_dep_env_key_value`.

## Motivation
Downstream `links` users (for example `hdf5-metno`) expect specific
`DEP_*` metadata keys (`DEP_HDF5_VERSION_*`, `DEP_HDF5_HAVE_*`).

## Testing
- `bazel --batch test //cargo/private/cargo_build_script_runner:test
--remote_executor= --remote_cache= --bes_backend= --noshow_progress
--color=no --curses=no`

`bazel test //test/cargo_build_script/metadata_dep_env:all` are end to
end integration tests

Closes #3876.