nix-prefetch-zip: unset $name

nix-shell leaks $name, breaking nix-prefetch-zip.

+1
+1
pkgs/build-support/fetchzip/nix-prefetch-zip
··· 15 } 16 17 18 argi=0 19 argfun="" 20 for arg; do
··· 15 } 16 17 18 + name="" 19 argi=0 20 argfun="" 21 for arg; do