fetchurl does not unpack, but we need it to support MacOS prefetch,
because the `--unpack` option to `nix-prefetch-url` does not handle
zips with multiple directories in them, and some upstream tarballs
switched to that.
GitHub's source tarballs are created in a non-deterministic way and the
order the entries are added is not stable.
In nixpkgs, there is fetchFromGitHub which works around this issue by
being a wrapper around fetchzip and since we're already using the full
URL to the corresponding GitHub archives, I switched to fetchzip instead
to keep the changes minimal.
Signed-off-by: aszlig <aszlig@nix.build>
