python3Packages.tensorflow: mark as requiring big-parallel

The Bazel build part of tensorflow builds can really use as many cores
as is available - to avoid blocking a small machine for literally hours
while it churns away, mark the build as big-parallel so we can schedule
it to run quickly on a machine with lots of cores.

pkgs/development/python-modules/tensorflow/default.nix

@@ -325,6 +325,10 @@ let
           addOpenGLRunpath "$lib"
         done
       '';
+
+      requiredSystemFeatures = [
+        "big-parallel"
+      ];
     };
 
     meta = with lib; {