rpmbuild: use dnf.conf/yum.conf automatically
    
    Let `mock` decide which configuration option we need to append to.
    Per discussion in
    https://lists.fedoraproject.org/archives/list/
    devel@lists.fedoraproject.org/thread/IRMM6UYHRWOI3YM6IE4DTTV4YWF3J7G2/
    
    The way this is implemented is only compatible with mock 2.0, therefore
    the 'Requires: mock >= 2.0'.