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/fetchgit/default.nix

@@ -19,6 +19,7 @@ in
 , # Shell code executed after the file has been fetched
   # successfully. This can do things like check or transform the file.
   postFetch ? ""
+, preferLocalBuild ? true
 }:
 
 /* NOTE:
@@ -66,5 +67,5 @@ stdenvNoCC.mkDerivation {
     "GIT_PROXY_COMMAND" "SOCKS_SERVER"
   ];
 
-  preferLocalBuild = true;
+  inherit preferLocalBuild;
 }