prevent roundstart arts on station z-level from being bombs

[rando212]

I think they should be kept is VERY funny when one gets set off super early although shitty for antags. One round on manta and a sci didn't know what art bombs were and with in 6 mins had set off a massive one destroying half the station warenting a shuttle call less then 10 mins in just to find out nukies were here and had to rush planting.