Fixed an issue where the rule engine was unable to access variables exported by `FOREACH` in the `DO` clause. 

  Given a payload: `{"date": "2023-05-06", "array": ["a"]}`, as well as the following SQL statement:
  ```
  FOREACH payload.date as date, payload.array as elem
  DO date, elem
  FROM "t/#"
  ```
  Prior to the fix, the `date` variable exported by `FOREACH` could not be accessed in the `DO` clause of the above SQL, resulting in the following output for the SQL statement:
  `[{"elem": "a","date": "undefined"}]`.
  After the fix, the output of the SQL statement is: `[{"elem": "a","date": "2023-05-06"}]`