Give Hydra binary caches a low priority

We only want to hit a Hydra binary cache if static caches don't
provide a binary.

src/lib/Hydra/Controller/Root.pm

@@ -202,7 +202,10 @@ sub nix_cache_info :Path('nix-cache-info') :Args(0) {
     $c->stash->{'plain'} = { data => 
         #"StoreDir: $Nix::Config::storeDir\n" . # FIXME
         "StoreDir: /nix/store\n" .
-	"WantMassQuery: 0\n"
+	"WantMassQuery: 0\n" .
+        # Give Hydra binary caches a very low priority (lower than the
+        # static binary cache http://nixos.org/binary-cache).
+	"Priority: 100\n"
     };
     $c->forward('Hydra::View::Plain');
 }