fix: regression where all args became passthrough

Fixes #475

tag.go

@@ -302,6 +302,7 @@ func hydrateTag(t *Tag, typ reflect.Type) error { //nolint: gocyclo
 		return fmt.Errorf("passthrough only makes sense for positional arguments or commands")
 	}
 	t.Passthrough = passthrough
+	if t.Passthrough {
 		passthroughMode := t.Get("passthrough")
 		switch passthroughMode {
 		case "partial":
@@ -311,6 +312,7 @@ func hydrateTag(t *Tag, typ reflect.Type) error { //nolint: gocyclo
 		default:
 			return fmt.Errorf("invalid passthrough mode %q, must be one of 'partial' or 'all'", passthroughMode)
 		}
+	}
 	return nil
 }