livemedia-creator: pass --nomacboot on non-x86_64
    
    See https://pagure.io/releng/issue/9601 . It seems the default
    is 'try to enable Mac booting' and we need to explicitly pass
    `--nomacboot` to turn it off. We only support x86_64 Macs for
    now, so we don't want to try and make live images bootable on
    Mac on any other arch.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>