Reimplement AZP job limiter for BuildKite

[fingolfin]

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. We noticed because a PR accidentally triggered a 1000 Buildkite jobs 😅

[vchuravy]

This was intentionally removed since Impetus to moving to buildkite was that we were having jobs that required bigger matrix jobs than was possible AZP

[giordano]

Larger matrix yes (a single package may have loads of artifacts), but the check is about the number of packages touched, that's rarely desirable to be very large.