nixos/tests: stdenv.lib -> lib

These must have been reintroduced by mistake after the deprecation.

nixos/tests/postfixadmin.nix

···
 import ./make-test-python.nix ({ pkgs, ...} : {
   name = "postfixadmin";
-  meta = with pkgs.stdenv.lib.maintainers; {
+  meta = with pkgs.lib.maintainers; {
     maintainers = [ globin ];
   };
 

nixos/tests/tigervnc.nix

···
 with import ../lib/testing-python.nix { inherit system pkgs; };
 makeTest {
   name = "tigervnc";
-  meta = with pkgs.stdenv.lib.maintainers; {
+  meta = with pkgs.lib.maintainers; {
     maintainers = [ lheckemann ];
   };