Rivian R1S in the Mojave Desert. Credit: Brooke Crothers Rivian EVs are expensive. But Rivian is trying to make them more affordable with hefty discounts. The R1S SUV is now the Irvine, Calif.-based EV manufacturer’s best selling model. It’s outselling the R1T pickup by about 2 to 1, according to Cox Automotive’s Electric Vehicle Sales…