treewide: simplify pkgs.stdenv.lib -> pkgs.lib

The library does not depend on stdenv, that `stdenv` exposes `lib` is
an artifact of the ancient origins of nixpkgs.

nixos/tests/gitlab.nix

@@ -5,7 +5,7 @@ let
 in
 import ./make-test-python.nix ({ pkgs, lib, ...} : with lib; {
   name = "gitlab";
-  meta = with pkgs.stdenv.lib.maintainers; {
+  meta = with pkgs.lib.maintainers; {
     maintainers = [ globin ];
   };