Follow-up #7237 (see last paragraph of #7237 (comment) )

I did not think that calling compute_runahead once per main loop would be a significant efficiency hit in inactive workflows because the method returns early if the base point hasn't changed. So in inactive workflows it really only does this:

base_point = min(self.active_tasks)

HOWEVER, it's certainly better not to call it at all if nothing is happening - so fair call @oliver-sanders and @MetRonnie

Note the change in task_pool.py (commit 2) is not necessary, it's just to separate out the soon-to-be-gone Cylc 7 compat bit, allowing the Cylc 8 case to be cleaner (and avoids a Python loop).

Check List

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• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
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• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.