ci: check semver on labelling PRs

[robjtede]

PR Type

CI

PR Checklist

• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] A changelog entry has been made for the appropriate packages.
• [x] Format code with the latest stable rustfmt.
• [x] (Team) Label with affected crates and semver status.

Overview

When labelling PRs with B-semver-<type:major|minor|patch>, trigger job checking semver compatibility for a release of <type>.

[robjtede]

Hmm... this random idea worked out well. It's a kind of interesting technique.

@JohnTitor you might be interested in this for other projects?

[robjtede]

Actually I should do some negative tests before we adopt this.

And address amendments to a PR being able to run the same job without needing to re-label it.