nix-shell -p is broken when using nixpkgs.lib.nixosSystem

[arianvp]

Describe the bug

It seems nix-shell -p stopped working in 24.05 for me.

Steps To Reproduce

1. Build a nixos VM using nixpkgs.lib.nixosSystem
2. Run nix-shell -p aws
3. Run nix-shell -p aws --show-trace and the stack trace becomes shorter

[ssm-user@ip-172-31-1-45:/var/log]$ nix-shell -p aws
error:
       … <borked>

         at «none»:0: (source not available)

       … while calling the 'import' builtin

         at «string»:1:18:

            1| {...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (aws) ]; } ""
             |                  ^

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: experimental Nix feature 'flakes' is disabled; use '--extra-experimental-features flakes' to override

[ssm-user@ip-172-31-1-45:/var/log]$ nix-shell -p aws --show-trace
error:
       … while calling anonymous lambda

         at «string»:1:1:

            1| {...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (aws) ]; } ""
             | ^

       error: experimental Nix feature 'flakes' is disabled; use '--extra-experimental-features flakes' to override

Expected behavior

nix-shell -p

works

Screenshots

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Additional context

Add any other context about the problem here.

Notify maintainers

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
output here

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[arianvp]

Weirdly enough I can not reproduce this in a local VM? It only seems broken in the AMI? did we break something with make-disk-image.nix ?

[arianvp]

Debugging further. Somehow this slipped in:

$ echo $NIX_PATH
nixpkgs=flake:nixpkgs:/nix/var/nix/profiles/per-user/root/channels

[arianvp]

This happened after I pushed a flake-built NixOS Config to my machine and it turns out we set this new NIX_PATH since #254405 landed. But that PR does not enable flakes. So you get this very obscure error message.

[Qyriad]

That PR is only supposed to apply when using a flake-based NixOS configuration, and nixpkgs.lib.nixosSystem is only defined in Nixpkgs' flake in the first place, so I am a little confused on how this could even come up.

[arianvp]

Because nixpkgs.lib.nixosSystem is just using flakes on the host system. But that doesn't mean the target system it builds has flakes enabled. (And by default it doesn't).

The PR unconditionally sets the setting whether flakes is enabled or not. I think we should make it conditional instead