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

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 154d6b1..d871792 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -213,6 +213,7 @@
         "git_repository": "https://github.com/googlecloudrobotics/core.git",
         "http_config": "https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/pipelines/cloud-robotics.yml",
         "pipeline_slug": "cloud-robotics-core",
+        "disabled_reason": "Disabled due to incompatibility with Bazel@HEAD.",
     },
     "Flogger": {
         "git_repository": "https://github.com/google/flogger.git",