Use task document instead of duplicate key to prevent generators from re-executing

Using a duplicate key to prevent generators from re-running means that we cannot TTL the collection that contains generators. Instead we should check the task collection. This may be as simple as adding an additional check to t.GeneratedTasks earlier https://github.com/evergreen-ci/evergreen/blob/6b269c26ba8894eb815ccb9feb4f849fda9b937f/units/generate_tasks.go#L84.