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>