Update mirror for move from BM to UBC

Stuart Prescottrequested to merge
stuart/piuparts:develop into develop

mirror.bm.d.o is not accessible from pejacevic after it was moved to UBC

(I have no idea if there are other places where this needs to be changed and how the change gets deployed; piu-slave-bm-a may also need to be removed from the setup.)

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