Jenkins no longer comments with the location of a staged website preview

[kennknowles]

Example PR where the staging succeeded but no comment: apache/beam-site#362

So you have to dig it out. And it seems the useful log message might have been removed. We should leave it, so there's a backstop.

Imported from Jira BEAM-3322. Original Jira may contain additional context.
Reported by: kenn.