Addressed PR comment from @andy31415.

Author: shaoltan-amazon

scripts/build/builders/host.py

@@ -314,7 +314,7 @@ def __init__(self, root, runner, app: HostApp, board=HostBoard.NATIVE,
                  minmdns_high_verbosity=False, imgui_ui=False, crypto_library: HostCryptoLibrary = None,
                  enable_test_event_triggers=None,
                  enable_dnssd_tests: Optional[bool] = None,
-                 chip_casting_simplified=None
+                 chip_casting_simplified: Optional[bool] = None
                  ):
         super(HostBuilder, self).__init__(
             root=os.path.join(root, 'examples', app.ExamplePath()),