Fix unportable code in pgbench.

Fix unportable code in pgbench.

The buildfarm points out that UINT64_FORMAT might not work with sscanf;
it's calibrated for our printf implementation, which might not agree
with the platform-supplied sscanf.  Fall back to just accepting an
unsigned long, which is already more than the documentation promises.

Oversight in e6c3ba7fb; back-patch to v11, as that was.