mark currently running build as blocking dependency

[joscha]

Similar to:

- trigger: "data-generator"
  label: ":package: Generate data"

which would trigger a new synchronous build of the data-generator pipeline, I'd like to mark an already running build of the data-generator pipeline as a blocking step, e.g. something like:

- triggered: "data-generator"
  build_id: c0ffee-c0ffee-c0ffee-c0ffee
  label: ":package: Generate data"

which would use that same display of a triggered synchronous build, but instead of starting a new one, it would use the given build_id and mark the build with build_id as the one that we are waiting for.

[nhooyr]

Would #497 be a more appropriate solution for your use case?