The primary aim of this PR is to improve the failure messages we get a little. Seeing the latest commit is often useless, and it's probably better to just get a direct link to a list of commits for that specific branch so we can see what's been committed since the previous nightly run.

This PR also makes one minor improvement in the use of the now-deprecated set-output syntax for workflows, upgrading it to use the new approach as per https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

PR checklist

• Tests written/updated, or no tests needed
• CHANGELOG_PENDING.md updated, or no changelog entry needed
• Updated relevant documentation (docs/) and code comments, or no
  documentation updates needed