Currently flutter/packages uses a custom autopublishing system, because it predates pub.dev support for autopublishing. We should instead migrate to publishing using one of the supported options, so that we are maintaining less custom process.

Since this would involve our outpublishing logic only handling up to the tagging, it should be easy to fix #92675 as part of this, since the non-trivial part there in our current tooling would be checking out a different tag on the fly.

I don't think we'll be able to use the GitHub Actions version, since the tagging would be done by a modified version of our existing autopublish workflow, and there are significant restrictions on triggering workflows from other workflows. That means we'd need to use the Google Cloud Build version instead.

@godofredoc Is setting up the GCB project and service account for this something the infra team could drive? (There's not a specific timeline on this; maybe we could plan for Q3?)