Trigger Release Artifact Generation on Publish

Trigger Release Artifact Generation on Publish

We previously triggered release artifact generation on release creation. We
sometimes observed that the action failed to run. I hypothesized that we were
hitting rate limiting or something. I just stumbled across [this documentat-
ion](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#release), which says:

> Note: Workflows are not triggered for the `created`, `edited`, or `deleted`
> activity types for draft releases. When you create your release through the
> GitHub browser UI, your release may automatically be saved as a draft.

This must have been what was happening. This commit therefore changes the
trigger to the `published` activity. This should be more reliable.

This does have the unfortunate side effect that artifacts won't be generated
or attached until *after* the release has been published, which is what I was
trying to avoid by using the `created` activity. Oh well.