Re-enable rules_jsonnet in Downstream CI (#1937) This reverts commit e2b6b06c39bba496a44b4aa0de4ce8225f138f7e. https://github.com/bazelbuild/rules_jsonnet/issues/173 was fixed by https://github.com/bazelbuild/rules_jsonnet/pull/179.
See https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md