Include cartopy downloaded resources in GitHub Actions cache

[jfrost-mo]

Fixes #646

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [ ] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [x] All tests and CI checks pass.
• [x] Added an entry to the top of docs/source/changelog.rst
• [ ] Conda lock files have been updated if dependencies have changed.
• [ ] Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• [x] Marked the PR as ready to review.

[github-actions[bot]]

[jfrost-mo]

I'm still a little concerned that there is a race condition if the documentation build finishes first, as then the cartopy resources won't be downloaded. Its probably fine though, as then we just fall back to what we had.

[jfrost-mo]

Eh, its probably fine. Worst case we can separate the docs cache from the other cache, but that would slow things down a bit.

[jfrost-mo]

Merging without review as this is a performance GH Actions tweak, and has no impact on CSET code.