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>

builder/kojid

@@ -3590,6 +3590,8 @@ class LiveMediaTask(ImageTask):
 
         if arch == 'x86_64':
             cmd.append('--macboot')
+        else:
+            cmd.append('--nomacboot')
 
         if b_append:
             cmd.extend(['--extra-boot-args', '\"%s\"' % b_append])