metropolis/test: fix %v in cases where we should use %w

Change-Id: I3ad73960ad6655332c9d223e13796ca410dca582
Reviewed-on: https://review.monogon.dev/c/monogon/+/3095
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/test/nanoswitch/nanoswitch.go b/metropolis/test/nanoswitch/nanoswitch.go
index 0bc65bd..b4ced56 100644
--- a/metropolis/test/nanoswitch/nanoswitch.go
+++ b/metropolis/test/nanoswitch/nanoswitch.go
@@ -187,7 +187,7 @@
 		Scope: netlink.SCOPE_UNIVERSE,
 	}
 	if err := netlink.RouteAdd(route); err != nil {
-		return fmt.Errorf("could not add default route: netlink.RouteAdd(%+v): %v", route, err)
+		return fmt.Errorf("could not add default route: netlink.RouteAdd(%+v): %w", route, err)
 	}
 	return nil
 }