Try again to make pg_isolation_regress work its build directory.

Try again to make pg_isolation_regress work its build directory.

We can't search for the isolationtester binary until after we've set
up the environment, because otherwise when find_other_exec() tries
to invoke it with the -V option, it might fail for inability to
locate a working libpq.  So postpone that step.

Andres Freund