cerbero: Never use cached cerbero git repo

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.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/376>