Add "cron" tests

[schneems]

It's helpful to see when "main" started failing as external dependencies change (such as a change in defaults).

[edmorley]

The builder repo's tests are run every day, and they include a smoke test that builds the Ruby getting started guide. We'll also be adding Direwolf CNB tests in the future.

What failure modes did you have in mind to catch with cron tests here?

[schneems]

👍🏻 That seems good enough

[schneems]

Actually I remembered why I wanted it: For the case where someone is contributing, but tests are failing. I want them to be able to check if main is failing as well or it's just their PR.

Basically: It's less for me and more for 3rd party contributors (which we don't have yet, but I would like). Plus I'm tagging this as a wanted contribution for a hackday, hoping that someone can make an easy-ish pr https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#schedule