openqa/workers: skip nbde-client on a64 workers
    
    They aren't encrypted, and it causes networking config problems.
    We'd like to solve the underlying problem but we don't know how,
    this is good enough for now. Also drop the workaround things
    again because they don't seem to help.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>