FIX SHOULD_GATE_ENTROPY condition

[hoodmane]

We can't turn it off if IS_TRACING because it needs to have the same setting when creating a snapshot as when using it.

[danlapid]

LBTM but I don't really understand what's behind this change here or what it causes.
If you feel like you need a review here I'd suggest tagging someone from the python team.

[hoodmane]

Thanks @danlapid. I added Dominik who reported the bugs this fixes.

[dom96]

For context, while testing package memory snapshots I ran into two issues:

• when tracing is enabled, which it is every time you use crossbow, the entropy code crashes the worker with "Cannot use random.seed() outside of request context"
• invalidating caches was only done for baseline snapshots, so they weren't properly invalidated for package snapshots which lead to errors like "ModuleNotFoundError: No module named 'index'" (where index.py is the main module in the worker)

This PR fixes those. Thanks @hoodmane!