Since the build jobs are no longer running in a regular pipeline,
the provision job no longer runs before to provide the app URL to
the extension build. Thus, we'll have to reconstruct it ourselves.

(This does result in one more place to keep up-to-date if the URL
structure ever changes, but since it explicitly lists `plaudit.pub`
it will probably be detected if the production domain ever changes,
and it's not as critical for review apps if _their_ URL structure
changes, and easy to fix once detected.)