ci: Simplify ci-os-only handling
Handle 'ci-os-only' occurrences in the .cirrus.star file instead of
.cirrus.tasks.yml file. Now, 'ci-os-only' occurrences are controlled
from one central place instead of dealing with them in each task.
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/
20240413021221.hg53rvqlvldqh57i%40awork3.anarazel.de
Backpatch: 15-, where CI support was added