rpmbuild: don't throw cleanup output to logs
    
    This was unnecessary output in each build log:
    INFO: mock.py version 2.0 starting (python version = 3.7.6)...
    Start(bootstrap): init plugins
    INFO: tmpfs initialized
    INFO: selinux enabled
    INFO: chroot_scan: initialized
    INFO: compress_logs: initialized
    Finish(bootstrap): init plugins
    Start: init plugins
    INFO: tmpfs initialized
    INFO: selinux enabled
    INFO: chroot_scan: initialized
    INFO: compress_logs: initialized
    Finish: init plugins
    INFO: Signal handler active
    Start: run
    Start: scrub ['cache']
    INFO: scrubbing cache for child
    Finish: scrub ['cache']
    Finish: run
    
    Merges: #1285