gitlab-ci: only run arch/pip job on final merged commits
gitlab-ci: only run arch/pip job on final merged commits

Each of these takes a while to run, and this one rarely catches
issues separately from the main jobs.

fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver