cerbero: Never use cached cerbero git repo

Sometimes the git repo cached on the docker image gets corrupted on Windows, in which case there is no way to clean things and resolve that error. For example: https://gitlab.freedesktop.org/bilboed/gst-plugins-bad/-/jobs/5517037

++ cp -a C:/cerbero/.git .
cp: cannot create regular file `./.git/objects/pack/pack-cbdde9dc4dd07bff6761e6715e20d7c0abe62b9e.idx': Permission denied
cp: cannot create regular file `./.git/objects/pack/pack-cbdde9dc4dd07bff6761e6715e20d7c0abe62b9e.pack': Permission denied

Always delete the cached git repo and copy a pristine copy from the image. This will also avoid accumulation of garbage in the git repo.