Adjust CI for multiple release branches

We'll have a new release branch soon (#520 (closed)). CI must be adjusted.

The goal should be, that we can make a merge request to the v20.05 branch and CI works just as well as if we had made the merge request towards the master branch.

aports-build is probably the most complicated one that needs to be changed; we probably need to resolve the branch to compare against from channel in the checked out branches' pmaports.cfg, and then read the channel branch from channels.cfg (git show origin-original/master:channels.cfg?).