[CI] Add explorer to the preview

Summary: Now that the preview build system is live, we can leverage this for all our websites. Let's add the explorer.

Test Plan:

./contrib/teamcity/build-configurations.py preview-explorer

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D13689