release-tools: Don't rely on `<nixpkgs>` anymore
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? null }:
+if pkgs == null then (builtins.throw "The `pkgs` argument needs to be provided to release-tools.nix") else
let
# Original `evalConfig`
evalConfig = import "${toString pkgs.path}/nixos/lib/eval-config.nix";
