Actions from forks fail

[Morl99]

The PR #24 showed that our workflow does not work when triggered from a branch that is owned by a fork. We should fix this to allow contributions from outside.

[Morl99]

The reason is, that forks by design will not have write access for the GITHUB_TOKEN, see: https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#permissions-for-the-github_token

I wonder what the best way forward will be. Using a PAT?

@mxmehl can you advise?