GitLab is currently timing out a lot, and the number of consecutive `git
clone` operations which the `build-sysext` job does is basically
guaranteed to fail at some point. I’ve tried one CI run 30 times in a
row without success.
Temporarily allow this job to fail until GitLab is fixed.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3251>