Preserve bazel repository cache in github actions (#14495)

Highlights:

- Trimmed down to just the repository cache for faster upload / download
- Made the cache key only include files that affect external
dependencies (since that's what the repository cache caches) -
MODULE.bazel, codex-rs/Cargo.lock, codex-rs/Cargo.toml
- Split the caching action in to explicit restore / save steps (similar
to your rust CI) which allows us to skip uploads on cache hit, and not
fail the build if upload fails

This should get rid of 842 network fetches that are happening on every
Bazel CI run, while also reducing the Github flakiness @bolinfest
reported. Uploading should be faster (since we're not caching many small
files), and will only happen when MODULE.bazel or Cargo.lock /
Cargo.toml files change.

In my testing, it [took 3s to save the repository
cache](https://github.com/siggisim/codex/actions/runs/23014186143/job/66832859781).