Enable parallel query on EXPLAIN ANALYZE (#4325)

It seems that we forgot to pass the revelant
flag to enable Postgres' parallel query
capabilities on the shards when user does
EXPLAIN ANALYZE on a distributed table.

src/backend/distributed/planner/multi_explain.c

@@ -1063,7 +1063,7 @@ worker_save_query_explain_analyze(PG_FUNCTION_ARGS)
 
 	INSTR_TIME_SET_CURRENT(planStart);
 
-	PlannedStmt *plan = pg_plan_query_compat(query, NULL, 0, NULL);
+	PlannedStmt *plan = pg_plan_query_compat(query, NULL, CURSOR_OPT_PARALLEL_OK, NULL);
 
 	INSTR_TIME_SET_CURRENT(planDuration);
 	INSTR_TIME_SUBTRACT(planDuration, planStart);