nixos/nix-channel: don't set nix-path

[nbraud]

Description of changes

Do not set an empty nix-path (in nix.conf) when ! nix.channel.enable.

Otherwise, nix will ignore the NIX_PATH environment variable, and by extension the nix.nixPath NixOS option.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
• Tested, as applicable:
  • NixOS test(s) : activation-nix-channel ;
• Tested compilation of all packages that depend on this change using nixpkgs-review
  Only nixos-install-tools would be rebuilt, but it is blacklisted in nixpkgs-review.
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Module updates) Added a release notes entry if the change is significant
• Fits CONTRIBUTING.md.

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[mkg20001]

Looks good, im just unsure if there was any reason for that default edit: seen commit message

[nbraud]

Thanks for the review <3

[r-vdp]

There was work ongoing regarding this also in #323613, @CyberShadow, @roberth.

That being said, I had this patch added to my nixpkgs for a long time now.

[CyberShadow]

@mkg20001 @nbraud I believe @roberth objected to making this change by itself: #273170 (review)

So, it may make sense to revert this pull request.

[roberth]

I've merged #323613 which implements the first mitigation

print a clear warning in the activation script if the channel data still exists in /nix/var/... and can therefore still be found by Nix