Re-enable upb in downstream
Re-enable upb in downstream

The issue has been fixed.
https://buildkite.com/bazel/bazel-at-head-plus-disabled/builds/491
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration