rpmbuild: define mock 'root' config for make srpm
    
    When 'root' is _not_ redefined, the caches (and other directories) are
    shared with the included default.cfg config file.
    
    During the recent testing of Red Hat Internal Copr instance, it happened
    several times in row that heavily re-used VM cached the root
    (root-cache.tar.gz) of default.cfg (symlink fedora-32-x86_64.cfg), and
    later re-used the cache for make_srpm method.  This failed because when
    restoring from the cache the chroot_additional_packages config option
    wasn't processed and the reqested dnf package was missing to build the
    sources.