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...

polkadot/node/service/src/lib.rs

@@ -909,9 +909,7 @@ pub fn new_full<OverseerGenerator: OverseerGen>(
 		slot_duration_millis: slot_duration.as_millis() as u64,
 	};
 
-	let candidate_validation_config = if is_collator.is_collator() {
-		None
-	} else {
+	let candidate_validation_config = if role.is_authority() && !is_collator.is_collator() {
 		let (prep_worker_path, exec_worker_path) =
 			workers::determine_workers_paths(workers_path, workers_names, node_version.clone())?;
 		log::info!("🚀 Using prepare-worker binary at: {:?}", prep_worker_path);
@@ -927,6 +925,8 @@ pub fn new_full<OverseerGenerator: OverseerGen>(
 			prep_worker_path,
 			exec_worker_path,
 		})
+	} else {
+		None
 	};
 
 	let chain_selection_config = ChainSelectionConfig {