Hello. I am tying to download nix declaration through the network from private gitlab installation. I found thant nixpkgs `fetchurl` supports `curlOpts` arguments which I could use to pass authorization token for gitlab, so this is my declaration:

```
let
  rev = "20190902.50";
  sha256 = "09330cmhrc2wmfhdj9zzg82sv6cdhm4qgdkva5ni5xfjril2pf14";
  token = "XXXXXXXXXXXXXXXXX";
  pkgs = import <nixpkgs> {};
in pkgs.fetchurl {
  url = "https://gitlab.example.com/api/v4/projects/90/repository/archive.tar.gz?sha=${rev}";
  curlOpts = "-H 'PRIVATE-TOKEN:${token}'";
  inherit sha256;
}
```

It builds successfully:

```
$ nix-build nixpkgs.nix
/nix/store/ans05wd64q2xd40fdd2wqqn5p5h5s581-archive.tar.gz?sha=20190902.50
```

But archive contents is not what gitlab sends. And `sha256` value not having any effect:

```
$ tar --list -f '/nix/store/ans05wd64q2xd40fdd2wqqn5p5h5s581-archive.tar.gz?sha=20190902.50'
metadata.gz
data.tar.gz
checksums.yaml.gz
```

Whats wrong about `nixpkgs.fetchurl`?