openqa/worker: disable aarch64-02 with a special worker class
    
    openqa-aarch64-02.qa is broken in some very mysterious way:
    https://pagure.io/fedora-infrastructure/issue/8750
    until we can figure that out, this should prevent it picking up
    normal jobs, but let us manually target a job at it whenever we
    need to for debugging.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>