Override DISCOVERY_VERSION in install script

Override branch name `master` with the correct release branch when installing

util/install/sandbox.sh

@@ -52,6 +52,7 @@ VERSION_VARS=(
   ANALYTICS_API_VERSION
   ECOMMERCE_VERSION
   ECOMMERCE_WORKER_VERSION
+  DISCOVERY_VERSION
 )
 
 for var in ${VERSION_VARS[@]}; do