In reply to @charles:computer.surgery
no idea what you're trying to ask here. the commands i posted should all be run within the live iso. the nix build command should be one that builds the system closure you intend to install. you could use some other command instead if you want like nixos-rebuild build

I mean which one is the correct?

nix --experimental-features 'nix-command flakes' build ./nix#nixosConfigurations.UwU.config.system.build.toplevel

Or

nix --experimental-features 'nix-command flakes' build /mnt/home/lag/nix#nixosConfigurations.UwU.config.system.build.toplevel

In reply to @charles:computer.surgery
because if both of those point to a copy of the exact same nix expressions then the result will be the exact same because that's kind of the point of nix

Any idea how to do this without IFD?

{ pkgs, inputs, ... }:
let
  inherit (inputs) lix;
  patchedLix = pkgs.stdenv.mkDerivation {
    name = "patched-lix-source";
    phases = [
      "unpackPhase"
      "patchPhase"
      "installPhase"
    ];
    src = lix;
    patches = [ ./0001-fix-sandbox.patch ];
    installPhase = "cp -r . $out";
  };
  versionSuffix = "-pre${builtins.substring 0 8 lix.lastModifiedDate}-${
    lix.shortRev or lix.dirtyShortRev
  }";
in
{
  config.nixpkgs.overlays = [
    (self: super: {
      lix = self.callPackage "${patchedLix}/package.nix" {
        inherit versionSuffix;
        stdenv = self.clangStdenv;
      };
    })
  ];
}

Oh yeah, I can just do this:

{ inputs, ... }:
let
  inherit (inputs) lix;
  versionSuffix = "-pre${builtins.substring 0 8 lix.lastModifiedDate}-${
    lix.shortRev or lix.dirtyShortRev
  }";
in
{
  config.nixpkgs.overlays = [
    (self: super: {
      lix =
        (self.callPackage "${lix}/package.nix" {
          inherit versionSuffix;
          stdenv = self.clangStdenv;
        }).overrideAttrs
          (old: {
            patches = (old.patches or [ ]) ++ [ ./0001-fix-sandbox.patch ];
          });
    })
  ];
}

@ersei:

good:

    "lix-module": {
      "inputs": {
        "flake-utils": "flake-utils",
        "flakey-profile": "flakey-profile",
        "lix": [
          "lix"
        ],
        "nixpkgs": [
          "nixpkgs"
        ]
      },
      "locked": {
        "lastModified": 1751240140,
        "narHash": "sha256-gbFOUtmkzzNAiHoUtySwbi+AVDgWOW8CpWmTmkoaQiI=",
        "rev": "ea74d925e650948d296fb85e4671fb0ce944f550",
        "type": "tarball",
        "url": "https://git.lix.systems/api/v1/repos/lix-project/nixos-module/archive/ea74d925e650948d296fb85e4671fb0ce944f550.tar.gz?rev=ea74d925e650948d296fb85e4671fb0ce944f550"
      },
      "original": {
        "type": "tarball",
        "url": "https://git.lix.systems/lix-project/nixos-module/archive/main.tar.gz"
      }
    },

bad:

    "lix": {
      "flake": false,
      "locked": {
        "lastModified": 1751631393,
        "narHash": "sha256-4CVRbeYExqIDpFH+QMZb5IeUGkP6kA/zHSuExYoZygk=",
        "type": "tarball",
        "url": "https://git.lix.systems/lix-project/lix/archive/main.tar.gz"
      },
      "original": {
        "type": "tarball",
        "url": "https://git.lix.systems/lix-project/lix/archive/main.tar.gz"
      }
    },

I have /no/ idea why that happened though, it appears that it somehow downloaded that tarball without recording the correct headers?