ci-package-builder: Do not re-run build-source on merge requests

When setting only: [ branches, merge_requests ] it means that two pipelines for the same commit are created when submitting a branch for merges: one for the branch, and a detached one for the merge request, see https://docs.gitlab.com/ee/ci/merge_request_pipelines/index.html

We only need the one running on the branch, at the moment we don't do anything fancy on merge requests.