Fixes bug where using Execution the Hive sources no longer append data.

The hack we do in the Hive parquet sources no longer works so this is a work around. The benefit this has is that we can now choose which sources are append and which one's aren't.
I don't know of any concrete reason why we might want to avoid these nested executions. Still, I wonder if you should be checking append earlier and only running in a nested execution if append is true.