Disable Cloud Robotics Core in downstream pipeline (#2858)

Disables Cloud Robotics Core in the downstream pipeline
(bazel-at-head-plus-downstream) due to incompatibility with Bazel@HEAD.


https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/5754#01a0a33f-d3fd-4d12-9e43-ae9348f83f94