Rework GPG check
    
    This follows a suggestion on fedora devel mailing list
    https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/LGQXK4EABUV4SR45BVP7WR2E6JNDU6JS/
    Main difference with the previous check is that this no longer rely on
    the default keyring, which means people running builds locally won't get
    Dan Nicholson's key added to their default GPG keyring.