Move _set_running_loop up the stack

[dandavison]

This is a draft PR, investigating where we can / should set/unset the running loop.

Currently it's set/unset on entry/exit of _run_once(). Is the purpose of that to prevent any execution of workflow coroutines at any other time during activate()?

If so: CI is green when we do the setting/unsetting at the top level in activate(): it would be great to have a failing test demonstrating the necessity of confining the execution to _run_once().