ci-package-builder: Allow for incomplete lfs pushes with sources too

From commit c6293e70 "Allow pushing incomplete LFS updates":

The update step doesn't have all LFS objects in pristine-lfs downloaded on purpose as they are not needed (and would slow things down). Which causes a problem with some git lfs versions (notably the one in buster) as it will fail to push if it doesn't have all relevant objects locally even if they do exist on the server.

Tell git-lfs that it's all fine and good and it should push incomplete anyway.

Do the same when pushing to the pristine-lfs-source branch, as the pipeline sometimes is failing with:

$ git push --follow-tags "${CI_AUTH_PROJECT_URL}" "${PRISTINE_SOURCE}:${PRISTINE_SOURCE}"
Unable to find source for object a3703b626b4707830c87be5369f51986263afda8fc8e6d7ebdf65be3fc7de0fe (try running git lfs fetch --all)
error: failed to push some refs to 'https://not_a_robot:[MASKED]@gitlab.apertis.org/tests/dash.git/'