ci-package-builder: Fix concurrent tag-release jobs

When landing two release MRs on the same project they may end up running their tag-release jobs at the same time. This can lead to conflicts:

• job 1: pulls the pristine-lfs-source branch
• job 2: pulls the pristine-lfs-source branch
• job 1: commit new sources to the pristine-lfs-source branch
• job 2: commit new sources to the pristine-lfs-source branch
• job 1: pushes new HEAD for pristine-lfs-source
• job 2: fails to pushes new HEAD for pristine-lfs-source since job 1 already updated it

See https://gitlab.apertis.org/pkg/base-files/-/jobs/579753 as an example of failed job.