Cache the data, or upload it as an artifact

[samcunliffe]

Collecting the data and creating the JSON file is very slow. It'd be helpful for local testing if we could grab a copy.

It might also be nice if it could be cached to speed up the build. I think it's getting re-written every time.

Definition of done

• Investigated caching the data JSON file(s).
• Add an upload step to the action to save the output from successful runs.