Task hosts should set TaskEnvironment instead of relying on task defaults

[AR-May]

Currently, task hosts rely on individual tasks to provide a default TaskEnvironment. The out-of-proc task host never assigns TaskEnvironment on IMultiThreadableTask instances. Today this only works because our built-in tasks initialize the property to TaskEnvironment.Fallback themselves. To improve reliability the task host should be responsible for initializing and setting the TaskEnvironment.