[CI] Cache bitstreams on PRs
@mfnch is already working on this.
Since Aug 19, 2026.
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Description
### Description
Use the [github actions cache](https://github.com/actions/cache) to cache and restore bitstreams on PRs.
The plan is to cache bitstreams generated on PRs, such that they can be reused in a subsequent run during code review.
Since the cache is scoped to the key, version, and branch this will only speed up PRs that run repeatedly during code review and will not affect other branches not the bazel cache which will still be used for PRs that don't change RTL.
The cache key will be generated by bazel once the PR https://github.com/lowRISC/opentitan/pull/31041 is merged.
The reference implementation for this is in the [mocha](https://github.com/lowRISC/mocha/blob/32818cb34909922f6aca375d61758ae63aa3e7da/.github/workflows/ci.yml#L131) repository.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Assessment
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