.gitlab-ci.yml: Fix bst2 image name
REL_NAME was a remaining variable name that was not fixed one rebase.
See error: https://gitlab.com/freedesktop-sdk/infrastructure/freedesktop-sdk-docker-images/-/jobs/257576277
The 17.0 major release is coming on May 16, 2024! This version brings many exciting improvements to GitLab, but also removes some deprecated features. We are introducing three breaking change windows during which we expect breaking changes to be deployed to GitLab.com. You can read more about it on our blogpost. The third breaking change window begins 2024-05-06 09:00 UTC and ends 2024-05-08 22:00 UTC.
REL_NAME was a remaining variable name that was not fixed one rebase.
See error: https://gitlab.com/freedesktop-sdk/infrastructure/freedesktop-sdk-docker-images/-/jobs/257576277