fetchurl: add user agent

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

pkgs/build-support/fetchurl/default.nix

@@ -95,7 +95,7 @@ assert sha512 != "" -> builtins.compareVersions "1.11" builtins.nixVersion <= 0;
 
 
 let
-
+  inherit (stdenv.lib) fileContents;
   hasHash = showURLs || (outputHash != "" && outputHashAlgo != "")
     || sha1 != "" || sha256 != "" || sha512 != "";
   urls_ = if urls != [] then urls else [url];
@@ -132,6 +132,8 @@ else stdenv.mkDerivation {
 
   impureEnvVars = impureEnvVars ++ netrcImpureEnvVars;
 
+  nixpkgsVersion = fileContents ../../../.version;
+
   # Doing the download on a remote machine just duplicates network
   # traffic, so don't do that.
   preferLocalBuild = true;