cerbero ci: Wipe in before_script because gitlab doesn't
Noticed in this job, which failed because we rebuilt on top of an existing builddir:
https://gitlab.freedesktop.org/nirbheek/cerbero/-/jobs/4772728
Noticed in this job, which failed because we rebuilt on top of an existing builddir:
https://gitlab.freedesktop.org/nirbheek/cerbero/-/jobs/4772728