Fix deploy condition in docs-publishing workflow (again)

[jameslamb]

Contributes to #115

This should fix the issues we've been having with docs builds not triggering when we want them to. Specifically, only deploying the docs site on:

• push of a new tag
• manually-triggered via the GitHub UI at https://github.com/rapidsai/legate-boost/actions/workflows/build.yaml

Notes for Reviewers

How I tested this

I've tried iterating here in this repo (see #176, #177, #178, #179, #180), but the feedback cycle is just too slow. So I created a personal repo just to test how GitHub Actions configuration like this works: https://github.com/jameslamb/gha-test

Found that the change introduced in this PR successfully implements the behavior we want.

[seberg]

Thanks, 🤞 that this works. The logic does look right to me, but then I never expected the older things to not do what we want either :).

[jameslamb]

This is the one @seberg ! We have to believe! 😂