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/networking-proxy.nix

@@ -12,7 +12,7 @@ let default-config = {
       };
 in import ./make-test-python.nix ({ pkgs, ...} : {
   name = "networking-proxy";
-  meta = with pkgs.stdenv.lib.maintainers; {
+  meta = with pkgs.lib.maintainers; {
     maintainers = [  ];
   };