Rebuild the object store's provisioner image too

It gained a Makefile target today; a target the rebuild does not run is
the stale image this file was just fixed for.

test/rebuild.test.ts

@@ -31,7 +31,9 @@ test("every image the lab runs is rebuilt, not only the control plane's", () =>
   const builds = planned({ MESH_LAB_MODULES: "/repo/control/examples/modules" });
   const images = builds.find((b) => b.what === "images");
   assert.ok(images, "no image build at all");
-  for (const target of ["image", "builder-image", "provisioner-image", "proxy-image"]) {
+  for (const target of [
+    "image", "builder-image", "provisioner-image", "objectstore-image", "proxy-image",
+  ]) {
     assert.ok(images.argv.includes(target), `${target} is never built, so the lab runs a stale one`);
   }
 });