Up by Union Square, at Cantina (580 Sutter), they’re doing something a little different for Cinco de Drinko this year: They’ll be making all their drinks, margaritas included, with the rustic forebear of tequila, mezcal. Not everyone likes mezcal! But this should scare away the amateurs, and that’s a good thing. Drinks will be $6, and the bar opens an hour early Saturday, at 4 p.m.