Workaround mock-core-configs not being updated for F39 branch
    
    mock still thinks the releasever for Rawhide is 39, which causes
    it to use the wrong GPG keys and not be able to install packages.
    This overrides that setting in our mock config file, until
    mock-core-configs is updated in the distro.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>