add nix-prefetch-zip

This adds a `nix-prefetch-zip` utility that matches the behavior of `fetchzip`.
@jwiegley Unlike the other scripts, it's not actually used by any of the `fetch*` builders, so I figured I'd set it of on its own.

On second thought, though, I think it makes more sense to follow the same pattern as the others - I've updated this PR with that change.