Previously, we allowed this, but such triggers might query the table to
insert into and act differently if the tuples that have already been
processed and prepared for insertion are not there, so disable it in
such cases.

Back-patch to v14 where batch insert was added.

Discussion: https://postgr.es/m/CAPmGK16_uPqsmgK0-LpLSUk54_BoK13bPrhxhfjSoSTVz414hA%40mail.gmail.com