Disable Buildfarm in downstream pipeline (#2856)

Disables Buildfarm in the downstream pipeline
(bazel-at-head-plus-downstream) due to persistent downstream test
failures.


https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/5754#01a0a33f-d8fb-472d-a307-17621f6f7084

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 91b85fc..154d6b1 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -191,6 +191,7 @@
     "Buildfarm": {
         "git_repository": "https://github.com/buildfarm/buildfarm.git",
         "pipeline_slug": "buildfarm-farmer",
+        "disabled_reason": "Disabled due to persistent downstream test failures.",
     },
     "Buildtools": {
         "git_repository": "https://github.com/bazelbuild/buildtools.git",