Add `nix.nixPath` and `nix.keepOldNixPath` options

[Enzime]

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Description

This change is useful as it means flake users don't need to use channels and can just manage it through their flake configs:

{ inputs, ... }:

{
  nix.nixPaths = [ "nixpkgs=${inputs.nixpkgs}" ];
}

By default, the system value for $NIX_PATH is kept as a fallback. To completely override the system value for $NIX_PATH:

nix.keepOldNixPath = false;

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all.

• [ ] Test cases updated/added. See example.

• [ ] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [ ] Added myself as module maintainer. See example.

  • [ ] Added myself and the module files to .github/CODEOWNERS.

[berbiche]

I think this might clash with other places where Home Manager overrides the NIX_PATH? I'm not familiar with the shell code and all that so cc @rycee

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[stale[bot]]

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If you are the original author of the PR

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[Enzime]

@berbiche I had a look through the references to NIX_PATH in the home-manager repo and it seems like it should be fine. This PR is basically just a way to control the environment variable to match the nixpkgs option.

[ambroisie]

Is there an update for the status of this MR?

[Enzime]

Superseded by #4031