python3Packages.optuna: mark as broken.

This doesn't compile. It's also very out-of-date (over a year old), and
has been broken by various library upgrades as well.

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

@@ -84,6 +84,7 @@ buildPythonPackage rec {
   '';
 
   meta = with lib; {
+    broken = true;  # Dashboard broken, other build failures.
     description = "A hyperparameter optimization framework";
     homepage = "https://optuna.org/";
     license = licenses.mit;