ci: add an annotation that tells which repo, pr and other informations at the end of every build.

Created by: jhchabran

Presently, it's not very convenient to identify which repo was cloned and which PR is running. While it's rather obvious on the main repo, it's not at all the case on other pipelines.

Buildkite UI should provide this, but it's a few clicks away, which is adding some unnecessary friction when you're resolving an incident and jumping around pipelines.

A possible solution would be to create a BK plugin that does this automatically.