infra: Experiment with per-builder description_html on staging.

Testing what a description will look like on Milo for a staging builder.

Bug: None
Change-Id: I22cb9374ba60bdddc24879567d3abe6890eb68dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355275


Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798173}