[ticket/13232] Use a cache var with _ to force TTL and shuffle cron tasks

PHPBB3-13232

phpBB/phpbb/cron/manager.php

@@ -72,7 +72,8 @@ class manager
 	* @return \phpbb\cron\task\wrapper|null
 	*/
 	public function find_one_ready_task()
-	{
+    {
+        shuffle($this->tasks);
 		foreach ($this->tasks as $task)
 		{
 			if ($task->is_ready())