Enable keepcache in koji dnf conf
Supermin needs access to the pristine RPMs from which the buildroot
is installed, in order to grab the /etc (configuration) files into
the libguestfs appliance (the binaries etc are not baked into the
appliance for obvious security/maintainability reasons).
For years we got these from the yum cache, but problem: dnf defaults
to keepcache=0 and so doesn't save these RPMs.
Therefore enable keepcache like yum.
From https://fedorahosted.org/rel-eng/ticket/6274
Signed-off-by: Dennis Gilmore <dennis@ausil.us>