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.

Inizia con il workflow pr-trigger.yml e confrontalo con build-numpy.yml, quindi esamina la modifica al workflow introdotta da #169. Aggiorna le condizioni del trigger in modo che le modifiche relative esclusivamente alla pubblicazione non avviino build, mentre le altre modifiche agli script continuino a farlo, e verifica il comportamento risultante del workflow.