Run build-source on tags as well

This addresses the broken release detection when uploading to OBS since CI_COMMIT_TAG is present only when building tags and that's what the ci-buildpackage uses to detect whether it must mark the uploaded package as UNRELEASED and append a ~ to the version.

1. I pushed commits and the tag to the apertis/v2019pre branch
2. the pipeline got started for the branch, not for the commit
3. CI_COMMIT_TAG is thus not set
4. the scripts adds UNRELEASED to debian/changelog and appends ~ to the version, assuming it is a pre-release push instead of uploading a proper release version to OBS
5. sadness ensued

See the output of the job for a gvfs update.