Form submit POST renders page before submit handler is called
You need to add a submit callback that will handle the redirect:

 $form['#submit'][] = 'mymodule_search_callback'


And in that callback:    

function mymodule_search_callback() {
   $form_state['redirect'] = current_path() . and your urlencoded serch term;
}


Messing with $form['#action'] is not the right way to do it.