scan: exclude Cylc 7 workflows from scan results

[oliver-sanders]

An unintended side-effect of #4506 is that cylc scan is no longer able to tell between A Cylc 7 workflow that is running under Cylc 8 in compatibility mode and one that is running under Cylc 7.

This PR uses the log/workflow directory as a marker to exclude Cylc 7 workflows from scan results.

Note this fix also impacts the UIS which currently displays Cylc 7 workflows.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.

[MetRonnie]

Unfortunately, in my testing this merely makes a Cylc7 workflow that used to show up as myflow show up as myflow/log/suite now, when running cylc scan --state=all

[oliver-sanders]

The log/workflow/log dir was tricking scan because it kinda looks like a flow itself.

Needed to change dir_is_flow to cover three cases:

• It's a compatible workflow (show it in scan results).
• It's an incompatible workflow (don't show it in scan results but also don't descend further looking for workflows).
• It's not a workflow (descend further looking for workflows).

[MetRonnie]

Might be worth repeating the type annotation here so that nobody removes the is_flow is False check below in an effort to tidy

Suggested change

	is_flow = dir_is_flow(contents)
	is_flow: Optional[bool] = dir_is_flow(contents)

[datamel]

Checked out and tested.
I initially had a problem with this (a consequence of not tidying my run directories 🤦 ), I had a log directory from a previous run in the directory above my run dir which prevented it from showing. I suspect scan is going to need to be thought of when doing #4477, as this will (I think) potentially legitimize this scenario.