0cb623c3d9
It would be nice to be able to track Nix requests. It's not trustworthy, but can be helpful for stats and routing in HTTP logs. Since `fetchurl` is used so widely, we should "magically" get a UA on `fetchzip`, `fetchFromGitHub`, and other related fetchers. Since `fetchurl` is only used for fixed-output derivations, this should cause no mass rebuild. User-Agent example: curl/7.57.0 Nixpkgs/18.03