Using master of forks is dangeours

[koppor]

1. We update master in a GitHub actions "cron job".
2. The master of forks is also updated
3. The updates of the forks do NOT happen at the same time
4. That leads to different commit ids at each master branch
5. That leads to strange pull requests, when conributors merge master instead of upstream/master (strange behavior e.g. at mEDRA DOI fetcher implementation. #6641 (comment) and Add testing interface, including a set of capabilities to tests for #6687 (review))

Solution possibilities:

1. Guard the GitHub action by checking whether we are on JabRef/jabref
2. Have the CSL update happening at another branch and merge the branch using a pull request to our GitHub repository
3. Ensure that all documantation says upstream/master - also in our manual pull request comments.

Think, we should go for option 1, since we cannot ensure that all references to master are prefixed by upstream/master.

[Siedlerchr]

Something like this should work: if: github.repository == ‘JabRef/jabref’