ci: Use rsync for initial nfsroot population on cheza.

ci: Use rsync for initial nfsroot population on cheza.

rm -rf and then copying over all the contents again is a waste of time
when we'll almost always be using the same rootfs.  Saves about 30s of job
time.

Closes: #3065
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5266>