tests/functional/nix-shell.sh: Fix Polo test for VM test

It is unclear to me why this worked when not in a VM test, but the
explanation would be in the part of nix-shell we're getting rid of
with the devShell attribute.

tests/functional/shell.nix

@@ -56,6 +56,7 @@ let pkgs = rec {
   # See nix-shell.sh
   polo = mkDerivation {
     name = "polo";
+    inherit stdenv;
     shellHook = ''
       echo Polo
     '';