Carne Asada Tacos

Name: Carne Asada Tacos
Ingredients: 1 1/2 teaspoons cumin seed; 4 serrano chilies, halved (seeds removed for less heat); 2 cloves garlic; 3 tablespoons vegetable oil, plus more for brushing the grill and scallions; 2 tablespoons light brown sugar; Juice of 4 limes (about 1/3 cup), plus lime wedges, for serving; Kosher salt and freshly ground black pepper; 2 pounds skirt steak (halved, if needed, to fit on the grill); 2 bunches scallions; 24 white corn tortillas; Guacamole, for serving; Pico de gallo, for serving