Fix local.hex --if-missing (#6576)

Author: wojtekmach

lib/mix/lib/mix/tasks/local.hex.ex

@@ -35,7 +35,7 @@ defmodule Mix.Tasks.Local.Hex do
 
     should_install? =
       if Keyword.get(opts, :if_missing, false) do
-        Code.ensure_loaded?(Hex)
+        not Code.ensure_loaded?(Hex)
       else
         true
       end