update cache key to de-poison sample failed builds: https://buildkite.com/bazel/google-bazel-presubmit/builds/85988#0192fe72-8b8d-48e6-84c2-79a20e0f7e46 https://buildkite.com/bazel/google-bazel-presubmit/builds/85982
See https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md