Run CI in merge group, only on changes sent to main

[jakebailey]

This repo is particularly sensitive to PR merge races breaking things, since so much is generated and modified.

This PR updates the CI job to also be able to run in a merge group, with the intention that we could enable merge queues here that precheck PRs before they hit main, preventing stale PRs and races from getting merged.

This is not a complete change yet; we don't "require" any particular job to pass, which will require some changes in the repo settings and workflow here.