[migration] set a cnxset before process_script()
Not pretty, but I don't have a better idea right now and this at least
ensures migration scripts get run in the expected environment and can
access sql stuff as before.
for table, column in [
('transactions', 'tx_time'),
('tx_entity_actions', 'tx_uuid'),
('tx_relation_actions', 'tx_uuid')]:
repo.system_source.create_index(session, table, column)
commit()