ci-package-builder: Don't freeze MRs targeting secondary branches

Rework once again the freeze-stable-branches job to let people submit merge requests to secondary branches.

For instance if there's a topic branch named topic/whatever and a MR is submitted from a wip/user/foo branch we don't want to complain.

We only want to complain once topic/whatever is submitted to merge to apertis/v2020 rather than going through apertis/v2020-updates.