Guy Fieri is heating up Flavortown with a sweaty, shirtless selfie.The 55-year-old “Diners, Drive-Ins and Dives” host posted the photo from a sauna on Friday as he prepares for his son Ryder’s 18th birthday on New Year’s Eve. “Last workout of the year followed up with hot sauna and cold plunge. Hope everyone has great holidays!!” he wrote.The restaurateur’s signature bleach-blond hair was slicked back with sweat and sticking up on the sides and on top of his head.