test niri config without rebuilding os

flake.nix

# nix run --offline path:.#test-niri-config
{
  inputs = {
    nixpkgs.url = "nixpkgs/nixpkgs-unstable";
  };
  outputs = {nixpkgs, ...}: let
    system = "x86_64-linux";
    variable = "example";

    pkgs = import nixpkgs {inherit system;};
    config = pkgs.writeText "niri.config" ''
      // ${variable}
    '';
  in {
    packages.${system}.test-niri-config = pkgs.writeShellScriptBin "test-niri-config" ''
      ${pkgs.niri}/bin/niri -c "${config}"
    '';
  };
}