Add settings for `nix<->stack` integration.

[aloiscochard]

In order for stack to work properly in a NixOS environment, it is (sadly) required to pass a --nix flag every-time it is called.

Which mean that, at the moment, if you try to run codex in a stack project on NixOS, it will initially fail.

A workaround, is to edit the project stack.yaml and add nix: enable: true, which is sub-optimal as one might one to keep that only for his local project (and not pollute remote with that change).

Ideally we would like a "stackNixEnable: true" in .codex that will automatically add --nix on every stack call.