Add git mirror for tarball creation in Kubernetes

[gctucker]

WIth the new API & Pipeline deployment, the kernel source tarballs are being generated in a Kubernetes container. The first time this happens, a new git repository is being created from scratch which can take a while. Subsequent tarballs should happen more quickly but it's still a limiting factor, and the pod may need to be restarted every now and then.

We may also move tarball creation to a plain job rather than a pod, in which case it would definitively be recreated for every kernel revision. Or we may have replicas of the pod getting created dynamically.

To avoid this issue, one option is to have some kind of git mirror available. This could be a read-only persistent volume in the cluster with a Pod run periodically to update it. There's already support in kernelci.build.update_repo to have a reference repository which can be a mirror, to speed things up.