Between classes and rehearsals, Ryan Walker, 14, estimates he spends more than 20 hours a week dancing ballet.For the second year in a row he has been cast as the Nutcracker in Sierra Nevada Ballet’s production of “Peanutcracker: The Story in a Nutshell.” But his dedication to the art extends beyond the stage. In pursuit of his Eagle Scout rank, Walker is rebuilding the set for the performance. “They’ve been talking about it for a few years,” he said. “I figured this would be a good way to help benefit everybody.”Walker is focusing on the Christmas tree, which serves as …