nixos-config/modules/common/nix.nix

{
  pkgs,
  lib,
  config,
  ...
}:
with lib;
{
  options.allowUnfreePackages = mkOption {
    type = types.bool;
    default = false;
    example = true;
    description = ''
      Whether the system can use unfree derivations.
    '';
  };

  config = {
    nix = {
      settings.sandbox = true;

      package = pkgs.nixVersions.stable;

      extraOptions = ''
        experimental-features = nix-command flakes
      '';
    };

    nixpkgs.config.allowUnfree = config.allowUnfreePackages;
  };
}