Add gzip decompression support

[ocharles]

The following is enough to reproduce:

$ nix-build -E 'builtins.fetchTarball {name = "nixpkgs"; url = "https://github.com/NixOS/nixpkgs/archive/5f7eae4bbb15ce7fa4da1984fd6ba5be7af50952.tar.gz"; sha256 = "0pqhvq66s9z7pzscmz3h9vffybalvdnijw1z5c5kkx0c9d6dnjvr";}'
unpacking 'https://github.com/NixOS/nixpkgs/archive/5f7eae4bbb15ce7fa4da1984fd6ba5be7af50952.tar.gz'...
error: numeric field did not have utf-8 text: n����}#� when getting cksum for �

I am running NixOS with:

{
  nix = {
      systemFeatures =
        [ "benchmark" "big-parallel" "kvm" "nixos-test" "recursive-nix" "nix-command" ];

      extraOptions =
        ''
        experimental-features = recursive-nix nix-command
        '';

      package =
        let
          src =
            pkgs.fetchFromGitHub
              { owner =
                  "NixOS";

                repo =
                  "nix";

                rev =
                  "ac2bc721d84f3c6c214212e41808a45fa792c6e8";

                sha256 =
                  "1y5r9g7x244gvh511xkxqhcy1zydyx7rp38lzd5dnra5h9ja5gk0";
              };

        in
        ( import "${src}/release.nix" { nix = src; officialRelease = true; } ).build.x86_64-linux;
    };
}

[ocharles]

Removing the package override the example works, so it seems to be a regression.

[edolstra]

Probably caused by #2748.

[edolstra]

Oh yeah:

            // FIXME: add .gz support

:-)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/unable-to-use-gzip-in-derivation/12173/1