ci: include hints how to install OS packages.
This is useful for patches during development, but once a feature is merged,
new libraries should be added to the OS image files, rather than installed
during every CI run forever into the future.
Author: Justin Pryzby <pryzbyj@telsasoft.com>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/
20220310220611.GH28503@telsasoft.com