python3.tests.nixenv-virtualenv: Fix on darwin

The virtualenv mode is trying to be smart and checks whether it can
write to $out, which for some reason fails on darwin.

> virtualenv: error: argument dest: the destination . is not write-able at /nix/store

But what does work is creating it in the pwd and moving it to $out,
because mv just does what it is told.

pkgs/development/interpreters/python/tests.nix

@@ -43,7 +43,8 @@ let
       # Use virtualenv from a Nix env.
       nixenv-virtualenv = rec {
         env = runCommand "${python.name}-virtualenv" {} ''
-          ${pythonVirtualEnv.interpreter} -m virtualenv $out
+          ${pythonVirtualEnv.interpreter} -m virtualenv venv
+          mv venv $out
         '';
         interpreter = "${env}/bin/${python.executable}";
         is_venv = "False";