prefer-fetch-remote: an overlay to fetch on remote builders

This is useful when running tools like NixOps or nix-review
on workstations where the upload to the builder is significantly
slower then downloading the source on the builder itself.

pkgs/build-support/fetchhg/default.nix

@@ -1,4 +1,11 @@
-{stdenvNoCC, mercurial}: {name ? null, url, rev ? null, md5 ? null, sha256 ? null, fetchSubrepos ? false}:
+{ stdenvNoCC, mercurial }:
+{ name ? null
+, url
+, rev ? null
+, md5 ? null
+, sha256 ? null
+, fetchSubrepos ? false
+, preferLocalBuild ? true }:
 
 if md5 != null then
   throw "fetchhg does not support md5 anymore, please use sha256"
@@ -18,5 +25,5 @@ stdenvNoCC.mkDerivation {
   outputHash = sha256;
 
   inherit url rev;
-  preferLocalBuild = true;
+  inherit preferLocalBuild;
 }