BandwidthTestHandler: don't try to compare to a reference build unless lhcb-master-mr

For the last few days there's been an error in the handler log for the periodic BW tests, see e.g. here. There's been a few upstream merges in LbNightlyTools that may have caused it, however, looking into the code, we're trying to compare to a reference build in lhcb-master slots, which doesn't make any sense. On the other hand, in the case of a lhcb-master-mr slot, this error does not appear, as can be seen in this log (unfortunately that handler failed soon after as the reference build itself (I think just a coincidence) didn't complete properly on that day.

TODO:

• I should first test that a lhcb-master-mr handler log is definitely clean before merging this.