[buildbot] Advertise a preview url is available

Summary: Create a new teamcity comment as a feedback to the caller if a preview is available.

Test Plan:

./contrib/teamcity/build-configurations.py check-buildbot

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13669