python3Packages.tensorflowWithCuda: move assert to meta.broken

`meta.broken` can be caught nicely by CI when running attrpath
generation for Eval. `assert` can not and requires ugly workarounds.

They both do the same for the user, though. Thus using the former.

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

@@ -46,9 +46,6 @@
 # - the source build is currently brittle and not easy to maintain
 # - the source build doesn't work on NVIDIA Jetson platforms
 
-# unsupported combination
-assert !(stdenv.hostPlatform.isDarwin && cudaSupport);
-
 let
   packages = import ./binary-hashes.nix;
   inherit (cudaPackages) cudatoolkit cudnn;
@@ -233,5 +230,7 @@ buildPythonPackage rec {
       abbradar
     ];
     badPlatforms = [ "x86_64-darwin" ];
+    # unsupported combination
+    broken = stdenv.hostPlatform.isDarwin && cudaSupport;
   };
 }