GitHub caches are immutable, so #5883 was wrong (caches were never updated). Goes back to the previous caching strategy but with a cleanup workflow to delete caches for merged/closed PRs. The cleanup code comes from the example in GitHub docs: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries
Gleb Mazovetskiy