pgbench: Move constant into format string

pgbench: Move constant into format string

As coded, the row data strings generated for pgbench_accounts' COPY in
the client-side data generation were always assigning 0 for one of its
attributes.  This simplifies a bit an upcoming patch to switch
client-side data generation of pgbench to use COPY for the teller and
branch tables, rather than individual INSERTs.

Author: Tristan Partin
Discussion: https://postgr.es/m/CSTU5P82ONZ1.19XFUGHMXHBRY@c3po