Prevent basement labs generating needless overmaps

[Venera3]

Summary

None

Purpose of change

As noticed by @jbytheway while profiling worldgen mandatory specials that fail to get placed force a new overmap to generate (and so on) until they get placed. Since the basement hidden lab has trouble spawning owing to its weird hybrid nature it triggered this mechanism frequently, leading to slowdown (and potentially some mission weirdness).

Describe the solution

Reduced the special's frequency to 99% (down from 100%), turning it technically non-mandatory.

Describe alternatives you've considered

Redirect the whole questline at non-hardcoded labs, bringing their wholesale removal nearer. That will have to happen one of these days, preferably when we have fleshed out mutable microlabs and a few more modular labs (not volunteering though).

Testing

None, profiling stuff is beyond my paygrade.

Additional context

While mission code can place specials on pregenerated overmaps the chances of successful placement are much lower than on newly generated ones (our maps are very busy, and large specials error out pretty frequently). This doesn't really fix that, but it should lead to less unexplored-but-generated overmaps and as such easier target placement for missions.

[jbytheway]

FWIW, I also fixed this in an essentially equivalent way. I think it makes sense. When I profiled it, it didn't make much difference, but it did make the profiling results much easier to interpret by removing recursion, so that was nice.

I am a little too preoccupied to PR my version of the fox, so I'm happy to see someone else's.