Fix CI_SERVER_URL usage as it changes on external instances

I introduced a new variable for this altogether instead of relying on CI_SERVER_URL.

The MR is ready for review. The downstream pipelines are running: https://git.drupalcode.org/project/gitlab_templates/-/pipelines/495616

Note that we don't have any external integration to run this on, but given that we are hardcoding the URL to the correct value, it should just work.

fjgarlin → credited heddn → .

This all looks good. Updated the IS with the actual new variable.

This was merged and a new tag was made, which is as well the new default-ref for all contrib.