Disable flatbuffer in downstream pipeline (#1741)

1.[ [Error: 'apple_common' value has no field or method
'multi_arch_split' with
BAZEL@HEAD](https://github.com/google/flatbuffers/issues/8095#top)](https://github.com/google/flatbuffers/issues/8095)
2. [Flatbuffers is failing with bazel@HEAD and should upgrade to the
latest rules_js.](google/flatbuffers#7992)

Author: sgowroji

buildkite/bazelci.py

@@ -186,6 +186,7 @@
         "git_repository": "https://github.com/google/flatbuffers.git",
         "http_config": "https://raw.githubusercontent.com/google/flatbuffers/master/.bazelci/presubmit.yml",
         "pipeline_slug": "flatbuffers",
+        "disabled_reason": "https://github.com/google/flatbuffers/issues/7992",
     },
     "Flogger": {
         "git_repository": "https://github.com/google/flogger.git",