Support buildkite beta public pipelines

[mvines]

Buildkite now offers a beta feature to mark pipelines as beta, which is a great start towards making this project obsolete: see
buildkite/feedback#137 (comment)

For buildkite public pipelines, ci-gate does not need to serve its own version of the buildkite log and thus can avoid overriding the buldkite github commit status URL. Unless the buildkite API provides a way to check if a pipeline is public, we’d likely just use another environment variable to whitelist pipelines as public.