As the French multi-instrumentalist known as FKJ boarded a plane early this year, he turned to his manager, Kate Cudbertson, and asked: “Why are we going to Bolivia, again?” The huge risk of filming a remote concert at Salar de Uyuni — the world’s largest salt flat, high in the Andes — was starting to sink in. But they both knew they might be on the verge of making something for the ages.