limit azure runs to main branch

Summary:
Building on all branches doesn't make sense, especially once we start pusing more branches.

PRs will still run it - though on our current plan we may find this unsustainable as well.

Test Plan: eyes

Reviewers: max, rexledesma, prha, dgibson

Reviewed By: dgibson

Subscribers: dgibson

Differential Revision: https://dagster.phacility.com/D9027