Bazel CI is flaky due to cloning buildkite docker plugin

Bazel Auto Sheriff often reports flaky jobs failing like:
```
$ git clone -v -- https://github.com/buildkite-plugins/docker-buildkite-plugin .
Cloning into '.'...
fatal: unable to access 'https://github.com/buildkite-plugins/docker-buildkite-plugin/': Failed to connect to github.com port 443: Connection timed out
```
https://buildkite.com/bazel/bazel-auto-sheriff-face-with-cowboy-hat/builds/90

Is there a way we could cache this repo to avoid cloning from github every time?

This is also the cause for https://github.com/bazelbuild/continuous-integration/issues/963, because the flaky error will prevent updating last green commit and a rerun on the failed job won't trigger the updating step again.

/cc @philwo