corepkgs/fetchurl: the 'system' argument can be optional

corepkgs/fetchurl.nix

@@ -3,7 +3,7 @@ with import <nix/config.nix>;
 # Argh, this thing is duplicated (more-or-less) in Nixpkgs.  Need to
 # find a way to combine them.
 
-{system, url, outputHash ? "", outputHashAlgo ? "", md5 ? "", sha1 ? "", sha256 ? ""}:
+{system ? builtins.currentSystem, url, outputHash ? "", outputHashAlgo ? "", md5 ? "", sha1 ? "", sha256 ? ""}:
 
 assert (outputHash != "" && outputHashAlgo != "")
     || md5 != "" || sha1 != "" || sha256 != "";