pgsql: ci: Simplify ci-os-only handling

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: /messages/by-id/20240413021221.hg53rvqlvldqh57i@awork3.anarazel.de
Backpatch: 15-, where CI support was added

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/60b64e6a31a21449f4db49017417c225ee322986

Modified Files
--------------
.cirrus.star | 40 +++++++++++++++++++++++++++++++++++++++-
.cirrus.tasks.yml | 21 ++++++++++-----------
2 files changed, 49 insertions(+), 12 deletions(-)