Problems with macos-latest GHA runners

[aplowman]

GHA macos-latest runners are now Apple silicon machines (macos-latest, macos-14).

Problems:

• Python 3.8 does not seem to be available on macos-latest. E.g.: https://github.com/hpcflow/hpcflow-new/actions/runs/8829973633/job/24242153315
• Python 3.12: problems importing numpy on macos-latest. E.g.: https://github.com/hpcflow/hpcflow-new/actions/runs/8829973633/job/24242153315, https://github.com/hpcflow/hpcflow-new/actions/runs/8866905949/job/24344895622

[aplowman]

Relevant to problem 1 (missing Python 3.8): https://github.com/actions/runner-images/issues/9770

[aplowman]

Fixed for now by fixing to macos-13.