Naw, I eat enough beef for you to have guilt-free leather.
Besides, the ecological impact of manufacturing is much smaller than that of use of a vehicle. The real question is, how much fuel will you burn hauling around the extra dead weight of all that leather?
When you opt for the leather interior, do you also get heavier soundproofing material, a third-row DVD player, a larger engine, less efficient gearing, and heavier chassis to haul all that luxo-stuff around?