need new flake.nix for Poseidon dependency

On NixOS:
```
error: > cargo build --release # in directory ././lake-packages/Poseidon
error: stderr:
could not execute external process 'cargo'
error: external command `cargo` exited with code 255
```

Start by inspecting the requested flake.nix and the lake-packages/Poseidon directory, then reproduce `cargo build --release` on NixOS. Confirm that the flake provides the needed external command and that the Poseidon build completes without the reported error.

Written by the indexing model from the issue text.