Allow concurrency groups for synchronous trigger steps

Setting concurrency_group is currently not allowed on steps that trigger a build in other pipelines.

I can understand why this is the case for async triggers, but not why it needs to happen for the sync triggers, as the step is blocking until the triggered build has been completed.

Our use case is that we have one pipeline that “wraps” builds in other pipelines, and want to make sure that the whole “sub” job (not only the steps in that job) is executed in the correct order, and without any parallelism.

This is the output from buildkite-agent pipeline upload when trying to create a trigger step in a concurrency group:

2019-01-09 17:24:36 ERROR  Unrecoverable error, skipping retries
2019-01-09 17:24:36 FATAL Failed to upload and process pipeline: POST[UUID]/pipelines: 422 Concurrency cannot be set for steps that trigger builds