Add caching of CHIANTI files to GHA

[samaloney]

PR Description

Use openastronomy GHA templates caching features to cache the CHIANTI files.

So what should ideally happen with this PR is that once it's merge to main it will run and generate the cache entries for each os with keys ending in refs/heads/main e.g. sunpy-data-linux-refs/heads/main then any subsequent run will restore the cache from these keys

AI Assistance Disclosure

AI tools were used for:

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used

Regardless of AI use, the human contributor remains fully responsible for correctness, design choices, licensing compatibility, and long-term maintainability.

[samaloney]

Since these should not change maybe use broader cache key sunny-data-<os> that way once cached should always be there - or maybe on update to main sunny-data-main-<os> that way they are updated once in while?

Can also maybe cache the same data for the doc builds?

[Cadair]

Is there not a github context var for OS name?

[samaloney]

Maybe I could use runner.os and have single cache block for the all the envs

[samaloney]

Nope doesn't have access to the runner info at that stage