pg_regress reported "Unix socket" as the default location whenever
HAVE_UNIX_SOCKETS is defined.  However, that's not been accurate
on Windows since 8f3ec75d.  Update this logic to match what libpq
actually does now.

This is just cosmetic, but still it's potentially misleading.
Back-patch to v13 where 8f3ec75d came in.

Discussion: https://postgr.es/m/3894060.1646415641@sss.pgh.pa.us