Clean caches for PR from a fork

The `ComPWA/action/clean-caches` action currently does not have sufficient rights to clean caches for PR from a fork, see for example [this log](https://github.com/ComPWA/ampform/actions/runs/4032705401/jobs/6932753859). Compare to [this run](https://github.com/ComPWA/ampform/actions/runs/4032797873/jobs/6932905391), which was triggered after a PR from a branch.