cylc clean: intelligent host selection

[oliver-sanders]

In the event of a 255 error, the cylc clean command will fail over onto another platform, but not onto another host?!

Not sure how this came about, more of a partially implemented feature than a bug, not a helpful behaviour.

[oliver-sanders]

Have taken a look in, regretably, it will be hard to resolve this in a bugfix without making the logic 🤮.

This is tangentially related to #5017 as the clean logic effectively creates it's own lightweight subproc pool in order to throttle CLI commands. Async'izing this would be a nicer way forward when implementing this logic as it would avoid the need to port bad-hosts type logic over.