leaned too heavily on his physical comedy schtick from “The Mask” to play a 77-year-old and was stuck in a mediocre sketch in which his DNA became combined with the vice presidential debate fly.But Carrey waved good-Biden to his failures in the latest “SNL” cold open that took aim at this week’s “dueling town halls” — with Biden speaking to voters and George Stephanopoulos on ABC and President Trump appearing on NBC with Savannah Guthrie.His Biden zingers, at least, tickled.A voter in the