CI: Ensure necessary repos are cloned with submodules.

[microbit-carlos]

Needed for this PR (which will stay as a draft PR for the time being):

• FFT example #291

But can be merged immediately, as it's a good CI update to have in the main branch anyway.

[github-actions]

Build diff

Base commit: 50eef908efdb099d8d7397329a84bb1695c77fc0
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/4556034772

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[microbit-carlos]

Okay, GitHub actions is a bit broken at the moment but this works and I want to unblock the CI for #291, so I'll merge now.