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.