Run git lfs fsck on pristine-* branches

This allow MRs targeting pristine-* to be merged since when only_allow_merge_if_pipeline_succeeds is set GitLab does not allow for MRs to be merged if their pipelines are empty.

For instance I temporarily hacked pkg/util-linux> to use this branch for its pipelines in order to make this MR green:

pkg/util-linux!21 (merged)

See this job:

https://gitlab.apertis.org/pkg/util-linux/-/jobs/2620133

I've now reverted the pkg/util-linux> configuration hack.