#144 Update action versions and implement checkout of fork for tests

[nils-herrmann]

No description provided.

[Michael-E-Rose]

ChatGPT4 suggest to provide a fallback in case commits go directly into the master branch:

        ref: ${{ github.event.pull_request.head.ref || github.ref }}
        repository: ${{ github.event.pull_request.head.repo.full_name || github.repository }}

If you could add that, then I'll accept the PR, and we'll see what happens.