CI: make sure cron jobs are not running on forks

Apparently cron jobs are triggered for forked repos, too, we need to make sure we don't create competing parallel queries (and extra load) on the servers.

It feels like that most forks are OK, old fork looks fine and the newest, too. And not sure how much of this affect is due to people opting in actions on their forks.

(I'm somewhat hesitant to a large scale nuking of running any actions on forks as it could be useful in case of PRs against someone else's branch etc, but if the numbers show that this is a significant issue, then I may just do that and force disable it for all but the main repo (well, cancel any workflows that started, we can't really properly disable actions centrally)

And once this is done and is working, we should do the same for pyvo, there are certainly unnecessary fork-cron jobs there.