apache-airflow: mark broken

It doesn't build and probably won't any time soon, as it still depends on pre-2.0 Flask.

pkgs/development/python-modules/apache-airflow/default.nix

@@ -243,5 +243,7 @@ buildPythonPackage rec {
     homepage = "https://airflow.apache.org/";
     license = licenses.asl20;
     maintainers = with maintainers; [ bhipple costrouc ingenieroariel ];
+    # requires extremely outdated versions of multiple dependencies
+    broken = true;
   };
 }