flake: remove obsolete crane nixpkgs override

[geri1701]

Summary

Remove the obsolete crane.inputs.nixpkgs.follows override. Crane no longer exposes a nixpkgs flake input, so the override only produces this warning:

warning: input 'crane' has an override for a non-existent input 'nixpkgs'\n```\n\n## Validation\n\n- `nix-instantiate --parse flake.nix`\n- `nix flake metadata --no-write-lock-file`\n- `nix flake check --no-build`