[prtester] improve prtester.py and prhtmlgenerator.yml for running in forks

[User123698745]

This PR will improve prtester.py and prhtmlgenerator.yml for running in forks. It makes the hard-coded repo name and base url to the new rss-bridge-tests repo dynamic/configurable (by using the repo owners name and an optional ARTIFACTS_REPO variable). The Upload tests job will be skipped when the secret RSSTESTER_ACTION is missing (which will always be the case for forks, by default) instead of always failing. Additional small fixes to the Upload tests job: It failed when the prs directory did not exist yet and it failed when there was no change to the html files.

[Bockiii]

Thanks for all the help with this!

Because I've run into this before and maybe you know how to do it:

If you re-run the test manually within github, I've had the behavior that it cant identify the github event number (because the run wasnt caused by a commit but by a manual re-run request) and thus, the github event number defaults to "none".

I'm not sure if you can recreate that and if so, have a fallback to use something else but the github event number.

[User123698745]

I could not recreate that behavior by re-running the pipeline or single jobs of the pipelines. github.event.number was always set, but to make sure I added a fallback to value none.