Preserve bazel repository cache in github actions

[siggisim]

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.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[siggisim]

I have read the CLA Document and I hereby sign the CLA

[bolinfest]

recheck

[bolinfest]

OK, all the Bazel steps succeeded over at #15924 so let's try this out!