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/sudo.nix

@@ -6,7 +6,7 @@ let
 in
   import ./make-test-python.nix ({ pkgs, ...} : {
     name = "sudo";
-    meta = with pkgs.stdenv.lib.maintainers; {
+    meta = with pkgs.lib.maintainers; {
       maintainers = [ lschuermann ];
     };