copr: don't disable yum/dnf caches
    
    We re-use the mock profile several times during one package build,
    and it doesn't make sense to re-download the repo metadata all the
    time.
    
    We have nowadays namespaced directory under /var/cache/mock
    per-build ID, so there's no risk that we'll use some very old
    caches because of 'max_age_days' or 'max_metadata_age_days' config
    options.