A beloved Nassau County crossing guard who was rammed into by an accused drugged driver has died — and the motorist was allegedly high on “street Xanax” at the time, prosecutors revealed Tuesday. Joshua Alvarado, 30, now faces upgraded charges of manslaughter for allegedly jumping a curb and mowing down 70-year-old John Miro as he was on-duty helping children get to school in Merrick, the Nassau County District Attorney’s Office said.