"Try Update Last Green Commit" step sometimes fails

bazel-remote's scheduled bazelci builds sometimes fail even though an identical build on the PR succeeded earlier. Here are two examples:
https://buildkite.com/bazel/bazel-remote-cache/builds/765#24c5f205-547a-42e7-b873-fc8ed5bdfcc5
https://buildkite.com/bazel/bazel-remote-cache/builds/691#59e8a1d9-95b0-416d-8fe9-23016e5ef3f2

The failing step/job is "Try Update Last Green Downstream Commit", which looks like it thinks two different steps were failing(?) in these two examples:
`Cannot update last green commit due to 1 failing step(s): go vet on :ubuntu: 18.04 (OpenJDK 11)`

`Cannot update last green commit due to 1 failing step(s): :cop: Validate .bazelci/presubmit.yml`