ci: Remove extra PG_TEST_EXTRA from NetBSD/OpenBSD
The PG_TEST_EXTRA environment variable is already set at the top level.
As of
3d1aec225, Meson tasks will use this by default, so there's no
need for another intermediate variable.
Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/flat/CAN55FZ0aO8d_jkyRijcGP8qO%3DXH09qG%3Dpw0ZZDvB4LMzuXYU1w%40mail.gmail.com