Fix bogus logic for checking executables' versions within pg_upgrade.

Somebody messed up a refactoring here.  As it stood, we'd check pg_ctl's
--version output twice for each cluster.  Worse, the first check for the
new cluster's version happened before we'd done any validate_exec checks
there, breaking the check ordering the code intended.

A. Akenteva

Discussion: https://postgr.es/m/f9266a85d918a3cf3a386b5148aee666@postgrespro.ru