Support for buildkite's group step

[luxe]

Any thoughts around supporting buildkite's group step? If its something we want bazel's CI to support, send some pointers and I will look into implementing it.

[fweikert]

It might be a nice UI improvement to group tasks by project in the https://buildkite.com/bazel/bazel-at-head-plus-downstream pipeline. My understanding is that all jobs within a group run in parallel, so it shouldn't be a functional change.