fetchurl: Eliminate pointless cross differences

pkgs/build-support/fetchurl/default.nix

···
-{ lib, stdenvNoCC, curl }: # Note that `curl' may be `null', in case of the native stdenvNoCC.
+{ lib, buildPackages ? { inherit stdenvNoCC; }, stdenvNoCC, curl }: # Note that `curl' may be `null', in case of the native stdenvNoCC.
 
 let
 
···
   # resulting store derivations (.drv files) much smaller, which in
   # turn makes nix-env/nix-instantiate faster.
   mirrorsFile =
-    stdenvNoCC.mkDerivation ({
+    buildPackages.stdenvNoCC.mkDerivation ({
       name = "mirrors-list";
       builder = ./write-mirror-list.sh;
       preferLocalBuild = true;

pkgs/top-level/all-packages.nix

···
   fetchhg = callPackage ../build-support/fetchhg { };
 
   # `fetchurl' downloads a file from the network.
-  fetchurl = makeOverridable (import ../build-support/fetchurl) {
-    inherit lib stdenvNoCC;
+  fetchurl = if stdenv.buildPlatform != stdenv.hostPlatform
+   then buildPackages.fetchurl # No need to do special overrides twice,
+   else makeOverridable (import ../build-support/fetchurl) {
+    inherit lib stdenvNoCC buildPackages;
     curl = buildPackages.curl.override (old: rec {
       # break dependency cycles
       fetchurl = stdenv.fetchurlBoot;