GitLab CI: only run the apt-snapshots-expiry job when relevant

I.e. when at least one of these conditions is met:

• We're on one of our main branches (stable, testing, devel).
• We're on a pipeline for a MR that modifies 1+ APT snapshot serial.

This will:

• save resources
• avoid awkward situations like the pipeline failing on master, or on a MR whose target is master (and that does not modify APT snapshots serials), because the snapshots used by that branch will expire: we don't really care if that's the case, and we don't want e.g. tech writers to be bothered in that case