frontend: put arch into multilib repo name
    
    This is needed to avoid the seemingly duplicate dnf repo download
    output like
    $ dnf update
    ...
    Copr repo for ping owned by praiskup  5.8 kB/s | 6.5 kB     00:01
    Copr repo for ping owned by praiskup  5.6 kB/s | 6.5 kB     00:01
    ...