Workflow should trigger on pull_request.closed, not all pull_request.
The GitHub actions cache is helpful to speed up CI jobs. Once a branch is closed, we don't need the cacche anymore so we should delete it so the disk space can be reclaimed. This workflow was copied from https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries.