Narrow `pr-trigger.yml` to ignore publish-only changes in workflows

After creating #169 , the build-numpy.yml workflow kicked off again despite no new version to build. This is expected due to how the existing checker workflows were designed, and the publish step wasn't going to run anyway since it wasn't on main, but it's wasteful to be running a build in this case. If it's simple enough, we should change the pr-trigger.yml workflow so that it only triggers builds when parts of the script other than the publish job are modified. This may be too complex to justify.

Alternatively, we could do some further rework (once the composite action in #169 is merged) to have it perform the publish steps in a "mock" mode, so that we're testing the publishing steps as much as possible too. It would mean letting the superfluous wheel builds to run though, without further tweaks.

Start with the pr-trigger.yml workflow and compare it with build-numpy.yml, then review the workflow change introduced by #169. Update the trigger conditions so publish-only changes do not start builds while other script changes still do, and verify the resulting workflow behavior.