CI: jobs might fail due to "fatal: detected dubious ownership in repository at '/builds/jvoisin/mat2'"

Besides others, https://0xacab.org/jvoisin/mat2/-/jobs/376679 failed due to this error.

That's due an change in upstream git, which is fixed in upstream GitLab, see https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/3538 for details. However, the fix is merged, but not yet released.