Add Zuul postflight publish job

This was added by default for other non-core plugins in [1], but since
we define our own project, we need to add the job ourselves.

[1] https://gerrit-review.googlesource.com/c/zuul/config/+/314842

Change-Id: I8f2df1f85345fea679c15d10f6eb0d92bd1c74b3