gp_max_csv_line_length is a session level GUC. When change it in
session, it affects statement like select * from <external_table>.
But it does not work for INSERT INTO table SELECT * FROM <external_table>.
For such statement, the scan of external table happens in a QE backend
process, not the QD.
This fix add GUC_GPDB_ADDOPT so that setting this GUC in session level
can affect both QD and QE process.