Point update_url at the updates.json instead of directly at the add-on

addon/manifest.json.tmpl

@@ -13,7 +13,7 @@
     "gecko": {
       "id": "email-tabs@mozilla.org",
       "strict_min_version": "57.0a1",
-      "update_url": "https://testpilot.firefox.com/files/email-tabs@mozilla.org/signed-addon.xpi"
+      "update_url": "https://testpilot.firefox.com/files/email-tabs@mozilla.org/updates.json"
     }
   },
   "background": {