Add clean GitHub action to verify if the clean folder is up to date for all PRs

[leszekhanusz]

In my previous PR #530, at first I forgot to update the clean folder with my modification by running tools/clean.py

Looking at other PRs, this seems like a common mistake.

This PR will add a --verify-only flag to the clean.py script which allows to only verify if the clean folder is up to date without modification, then a GitHub action is created to verify this for all PRs and push to the repo.