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/+/2355275Reviewed-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}

infra/config/dev/subprojects/chromium/ci.star

@@ -68,6 +68,7 @@ ci_builder(
 
 ci_builder(
     name = "linux-rel-swarming",
+    description_html = "Test description. <b>Test HTML</b>."
 )
 
 ci_builder(

infra/config/generated/cr-buildbucket-dev.cfg

@@ -96,6 +96,7 @@ buckets {
           test_results {}
         }
       }
+      description_html: "Test description. <b>Test HTML</b>."
     }
     builders {
       name: "linux-rel-swarming-staging"