tests.trivial: Avoid evaluation and ${pkgs.path} dep

> There is an issue in the test added by #123111.
> [it] introduces a dependency on the contents of nixpkgs,
> making every change evaluate with a different hash.

pkgs/build-support/trivial-builders/test/invoke-writeDirectReferencesToFile.nix

@@ -0,0 +1,4 @@
+{ pkgs ? import ../../../.. { config = {}; overlays = []; } }:
+pkgs.lib.mapAttrs
+  (k: v: pkgs.writeDirectReferencesToFile v)
+  (import ./sample.nix { inherit pkgs; })