Check for unsupported OS images in `plan` job, before the actual release is requested

An issue I keep running into when using `dist` is that when I push a tag to cut a release, the release fails because one of the images (most recently Ubuntu 20.04 and Mac OS 13) is no longer available on Github Actions.

Every time I have to investigate what went wrong, update `dist`, delete the tag and re-apply it. Aside from being annoying, this causes issues with other automation that expects a tag not to change.

Ideally I'd like `dist` not to pick unavailable runners in the first place, but if it can't then I'd like to know that I need to update `dist` before I actually cut a release and see it fail.

Start by tracing how dist's plan job selects GitHub Actions runners and how that result feeds the release request. Check how unavailable Ubuntu 20.04 and macOS 13 images are represented, then verify the plan step reports the problem before a release tag is cut. Done means unavailable images are detected early or the user receives a clear update instruction.

Written by the indexing model from the issue text.