prevent roundstart arts on station z-level from being bombs

[Mouse]

See title.  The announcement has helped a lot, but there's still inexperienced people doing artsci and we had a round recently where a power sink drained the power at the exact moment a massive artbomb was activated, resulting in a 10 minute shuttle call on account of not being able to space it in time.

I feel like it's a good compromise to allow for artifacts to still blow stuff up while making it less likely they'll blow stuff up early on in the round.