nixosTests.virtualbox: fix eval

There's no "lib" in scope here. The test uses "with pkgs.lib;", so
this failed to evaluate.

Fixes: 6672dde558ea ("treewide: use optionalAttrs instead of 'else {}'")

nixos/tests/virtualbox.nix

···
     destroy_vm_test1()
     destroy_vm_test2()
   '';
-} // (lib.optionalAttrs enableUnfree unfreeTests)
+} // (optionalAttrs enableUnfree unfreeTests)