Split out cron CI into separate workflow

[Cadair]

Describe the feature

Doing this should have a few advantages:

• You can trigger on a label being added: https://github.com/astropy/astropy/blob/main/.github/workflows/ci_cron_daily.yml#L13
• It's easier for projects to opt out (we could add an option for cron CI)

Proposed solution

No response