openqa-a64-worker04: cut worker count a little
    
    We seem to be getting lots of failures on compose tests, I think
    because of how many tests kick off all at once, because they
    seem to work OK when restarted one by one. Maybe this worker
    count is a bit too ambitious (though it used to work), let's
    try cutting it a bit.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>