pw_presubmit: Add file to indicate upload is done

Add new file to uploads—'upload_complete'—to indicate to other users of
the uploaded bucket that this job has finished uploading data.

Change-Id: I4b9a08a9a24ddfa14e36bf03eff1d5f7eb4a6882
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/27300
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Oliver Newman <olivernewman@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>