T8283: Add note about OBS_REBUILD_DISABLED CI/CD environment variable

The packaging pipeline will automatically rebuild the packages on OBS when a new pipeline is triggered. This is enabled by default since users may not have OBS access. To disable this behaviour, set the CI/CD environment variable OBS_REBUILD_DISABLED (i.e. to any value) when triggering a new pipeline.

Fixes: https://phabricator.apertis.org/T8283