Removed the static lookup code from critical path

[airportyh]

Why

We found that there's currently not a practical way to make the cacache disk available in deployments, so we are going to disable the static channel-based look up for now. Relying on REPLIT_LD_LIBRARY_PATH only will still be beneficial if the follow hypothesis is true:

• as long as the Nix channel of the entrypoint binary (python3 or node) is later than the Nix channel of a dynamically loaded shared object, the likelihood of breakage is low.

So test cases have shown this to be true. I plan to conduct some experiments to test this more.

Changes

1. Removed call to lookup_by_channel
2. Removed code for dynamic Nix channel detection.
3. Updated integration test to match

[ryantm]

Looks good. 🤞 that hypothesis holds!