sched/task: Fix wrong return value from nxspawn_open()

nxspawn_open() is expected to return "OK" when it success, but
it doesn't return it in case of executing dup2.
Because of this, the "Command as parameter" couldn't work with
Builtin Apps.

sched/task/task_spawnparms.c

@@ -128,6 +128,10 @@ static inline int nxspawn_open(FAR struct spawn_open_file_action_s *action)
         {
           serr("ERROR: dup2 failed: %d\n", ret);
         }
+      else
+        {
+          ret = OK;
+        }
 
       sinfo("Closing fd=%d\n", fd);
       nx_close(fd);