Discovered after adding [must_use_result] to Worker::with_active_host in #2500.

Adding the unwrap in HostTreePointer::ptr currently causes the test small_stop_time-shadow to fail, because the task to start a process is created and then discarded during setup, before any Host is set active.

I think to fix this we need to move process creation out of the primary Host setup, so that we can set the active Host.