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/wireguard/basic.nix

@@ -6,7 +6,7 @@ import ../make-test-python.nix ({ pkgs, lib, ...} :
   in
   {
     name = "wireguard";
-    meta = with pkgs.stdenv.lib.maintainers; {
+    meta = with pkgs.lib.maintainers; {
       maintainers = [ ma27 ];
     };