When mozharness overrides the buildbot properties with S3 URLs, they aren't written back out to the file, so they only have an effect while mozharness is running. At some point we'll want to add write_to_file=True so that they persist, though I'm not sure yet what side-effects this will cause.

I don't think this is relevant anymore, since we seem to be getting along fine without it and all the things are almost in Taskcluster anyway.