On master (after 700538) the old version's installed psql was used -
even when the old version might not actually be installed / might be
installed into a temporary directory. As commonly the case when just
executing make check for pg_upgrade, as $oldbindir is just the current
version's $bindir.

In the back branches, with --install specified, psql from the new
version's temporary installation was used, without --install (e.g for
NO_TEMP_INSTALL, cf 47b3c266), the new version's installed psql was
used (which might or might not exist).

Author: Andres Freund
Discussion: https://postgr.es/m/20190522175150.c26f4jkqytahajdg@alap3.anarazel.de