Task hosts should set TaskEnvironment instead of relying on task defaults

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`.