[V4] Using `ensure()` on a task spawned within the current scope raises `Error("halted")`

[cowboyd]

To reproduce, run the code snippet in this discussion https://github.com/thefrontside/effection/discussions/975#discussioncomment-12338862

Expected:

script delays for 2000ms while the sleeping tasks completes.

Actual:

Raises Error("halted")