lib/tests: Allow overriding pkgs independent of lib being tested

lib/tests/maintainers.nix

@@ -1,10 +1,11 @@
-# to run these tests:
-# nix-build nixpkgs/lib/tests/maintainers.nix
-# If nothing is output, all tests passed
-{ pkgs ? import ../.. {} }:
+# to run these tests (and the others)
+# nix-build nixpkgs/lib/tests/release.nix
+{ # The pkgs used for dependencies for the testing itself
+  pkgs
+, lib
+}:
 
 let
-  inherit (pkgs) lib;
   inherit (lib) types;
 
   maintainerModule = { config, ... }: {

lib/tests/release.nix

@@ -1,8 +1,17 @@
-{ pkgs ? import ../.. {} }:
+{ # The pkgs used for dependencies for the testing itself
+  # Don't test properties of pkgs.lib, but rather the lib in the parent directory
+  pkgs ? import ../.. {} // { lib = throw "pkgs.lib accessed, but the lib tests should use nixpkgs' lib path directly!"; }
+}:
 
 pkgs.runCommandNoCC "nixpkgs-lib-tests" {
-  buildInputs = [ pkgs.nix (import ./check-eval.nix) (import ./maintainers.nix { inherit pkgs; }) ];
-  NIX_PATH = "nixpkgs=${toString pkgs.path}";
+  buildInputs = [
+    pkgs.nix
+    (import ./check-eval.nix)
+    (import ./maintainers.nix {
+      inherit pkgs;
+      lib = import ../.;
+    })
+  ];
 } ''
     datadir="${pkgs.nix}/share"
     export TEST_ROOT=$(pwd)/test-tmp