rpmbuild: do not scrub mock caches
This is safe (we can drop the unique 'root' ID) because we can have at
most O(N) directories in /var/lib/mock for N enabled chroots for
corresponding copr project. That's because builds across different
projects don't share the same VMs (VMs and builds belong to sandboxes
and each project has different sandbox ID).
Each of the /var/lib/mock/<chroot> directories should be small enough,
but we still plan to implement #787.