hydra: avoid perl warning about undefined variable

hosts/hydra/default.nix

@@ -7,7 +7,6 @@ in
   imports = [
     ./hardware-configuration.nix
     ./network.nix
-    ./hydra.nix
     ./updater.nix
     ../../modules/c3d2.nix
   ];
@@ -135,6 +134,7 @@ in
         in
         ''
           binary_cache_secret_key_file = ${key}
+          compress_num_threads = 4
           evaluator_workers = 4
           evaluator_max_memory_size = 2048
           max_output_size = ${toString (5*1024*1024*1024)} # sd card and raw images