Reimplement AZP job limiter for BuildKite

When we were still using Azure Pipelines, there was code that would prevent a PR from creating too many jobs. This was apparently [lost in the migration to Buildkite](https://github.com/JuliaPackaging/Yggdrasil/commit/a1b9ad54254b7709878a12f9afe46dd8eb0137bf). We noticed because a PR accidentally triggered a 1000 Buildkite jobs 😅