[infra] Support posting results to staging in post_results_to_pubsub
[infra] Support posting results to staging in post_results_to_pubsub

This adds the option '-s' to select posting the results to the
staging system instead of the production system.

Change-Id: I990a39c7db3d0379935b69007ebc9477da5cc051
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151528
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>
Commit-Queue: Karl Klose <49bc0b76cffb0b10cd8c84f9cabc97a31ffe6343@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk