CI: Clean caches of closed PRs
Looks like we regularly go over the 10GB CI cache limit. One step that should help is removing CI caches from closed PRs

https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries
Can we test this somehow without merging? I guess we could change the `on` condition to make this PR clean the caches, but I think this PR is pretty innocuous. I'll merge for now and see how things go on main. I can revert if it causes any problems