ci-package-builder: Add the pull-mirror job for downstreams

The pull-mirror job is not used on the Apertis infrastructure but it provides a reference for downstreams that want to mirror Apertis repositories and fetch updates from them.

The job checks the version of the source package actually published in the APT repository and mirrors the git branches accordingly: this avoids pulling commits for versions that for any reason (like a build failure on OBS) have not been published in the APT repositories.