PVF: Fix external workers being required for non-collator full nodes (#7566)

* PVF: Fix external workers being required for non-collator full nodes

* Fix

* Make check more precise

> This might as well be if role.is_authority() as there aren't really any nodes
> that are both authorities and collators. But if there were a node that is an
> authority and a collator, it'd probably want to validate candidates (in its
> role as an authority).

* Just seeing if this fixes zombienet CI...