Fix MRs for backports that fail to upload

When creating a backport MR, for instance from apertis/v2023dev1 to apertis/v2022, the HEAD will have a release tag created when it landed on the apertis/v2023dev1` branch.

The presence of the tag causes the build-source job to exit early, but that meant that no _build artifacts directory is populated.

The upload job will not have anything to upload to OBS and will fail.

Extract the tagged source from the pristine-lfs-source branch in the build-source job as well to have it ready for upload.