Implement a cleanup command for nightly branches

[tomuben]

Background

Currently, we create new branches automatically, using a GH Actions, with the pattern rebuild/nightly_YEAR_MONTH_DAY_HOUR_MINUTE_SECOND for example rebuild/nightly_2022_06_21_01_37_06. The push event to that branches then triggers the nightly CI builds.We need to run a cleanup manually, periodically, in order to remove the zombie branches.

It would be better to also call a cleanup task, in the same GH Actions, which removes automatically nightly branches, older than 3 working days.

Acceptance Criteria

1. Implement a new command which:
   1. Finds all branches with the pattern rebuild/nightly_YEAR_MONTH_DAY_HOUR_MINUTE_SECOND which are older than 3 working days
   2. Removes the branches on Github