"Try Update Last Green Commit" regularly fails, but succeeds when retried

I noticed that the tip of the master branch in the bazel-remote repository often has a red cross because the "Try Update Last Green Commit" job has failed with error message `Cannot update last green commit due to 1 failing step(s): :cop: Validate .bazelci/presubmit.yml`, despite everything else being green. And when I retry the build, it turns green.

For example: https://buildkite.com/bazel/bazel-remote-cache/builds/1587#d6a00835-c7a9-4a07-a2b7-b7af22ff45eb

I can't tell if there's a problem with my yaml file (all the steps run and pass), or if this is a bazelci bug.

Start with .bazelci/presubmit.yml and the referenced Buildkite build, comparing the failed run with its successful retry. Determine whether the intermittent failure comes from the YAML configuration or the bazelci validation, and leave the update-last-green-commit job reliably passing.

Written by the indexing model from the issue text.