Add index to temp table

synapse/storage/schema/delta/54/delete_forward_extremities.sql

@@ -20,3 +20,4 @@ INSERT INTO background_updates (update_name, progress_json) VALUES
 
 DROP TABLE IF EXISTS _extremities_to_check;  -- To make this delta schema file idempotent.
 CREATE TABLE _extremities_to_check AS SELECT event_id FROM event_forward_extremities;
+CREATE INDEX _extremities_to_check_id ON _extremities_to_check(event_id);