make-tarball.nix: Allow passing in a different Nix

Used by the Nix tests (https://github.com/NixOS/nix/commit/9fff4925613d44ac12775ad8840709130b9498d1).

pkgs/top-level/make-tarball.nix

···
    also builds the documentation and tests whether the Nix expressions
    evaluate correctly. */
 
-{ pkgs, nixpkgs, officialRelease }:
+{ nixpkgs
+, officialRelease
+, pkgs ? import nixpkgs.outPath {}
+, nix ? pkgs.nix
+}:
 
 with pkgs;