Support schedule events

[ianlewis]

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule

I don't recall the reason why they were excluded. Was there some info missing from those events that we cared about?

/cc @MarkLodato @laurentsimon @kpk47

[MarkLodato]

I suspect the reason was to avoid any ambiguity about which specific version was built. While the GITHUB_SHA and GITHUB_REF are set to the default branch, it seems conceivable that builds check out arbitrary branches during the workflow. Then again, it's possible to do that for the other events too.

I have no objection to adding it. It seems that the semantics are reasonably clear.

[ianlewis]

Right. I think schedule events are triggered on the default branch at HEAD so we would be presuming it was built from there but I think you're right it's not unlikely that someone would build from an arbitrary commit. However, from a technical perspective this is the same as how all other events work.

I just ran into this as part of tests, so there isn't an end user need for this yet. I'll let it stew a bit before sending a PR to add it.