CI: PR reporting doesn't work when running from forks

[georgkrause]

If someones files a PR using their own fork, which is supposed to be the normal case, the PR reporting doesn't work because Actions in forks run with read only one-time codes.

There is a pretty neat way to work around this issue documented here: https://github.com/dorny/test-reporter?tab=readme-ov-file#recommended-setup-for-public-repositories

I think I want to adopt this to try if it works with our tooling as well.

[georgkrause]

Sadly its not easily possible to download artifacts from "foreign" workflow runs, so this isn't really a solution here, unless we add some manual API interaction, which then would need to be maintained. So for now, we just don't have PR comments with test results and coverage.