Revert "chromium: remove preferLocalBuild"

This reverts commit fdb5cf8107.

The reason I'm reverting this is that the implications this had on the
IO load of Hydra are fixed by c92dbffeac.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/applications/networking/browsers/chromium/source/default.nix

@@ -80,6 +80,8 @@ in stdenv.mkDerivation {
       "$main/content/browser/browser_main_loop.cc"
   '';
 
+  preferLocalBuild = true;
+
   passthru = {
     inherit version channel;
     plugins = fetchurl binary;