Build on PR and post-PR master branch only

We would like to perform build on PR, and on the master after a PR has been rebased or squashed (we generally avoid the GitHub “Create a merge commit” strategy). Superficially, this looks like it would work if the build pushed branches option is turned off. However this also prevents the master branch from being built after the PR has been merged. So this breaks a workflow that keeps track of the master branch’s health.

Is there a work around for this or can the configuration be made to work the way that it reads that it should work (master is always built after a PR)?