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
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Noticed in this job, which failed because we rebuilt on top of an existing builddir:
https://gitlab.freedesktop.org/nirbheek/cerbero/-/jobs/4772728