Review whether we're using any deprecated predefined CI_BUILD_* variables
As part of 16.0, a new breaking change will remove previously deprecated CI_BUILD_* variables - gitlab-org/gitlab#352957 (closed)
We should review if we might be affected by this deprecation and plan any required work. Please make sure to consider GitLab.com, Pre, Release, and Ops as part of this issue (or bring in other people to allow us to consider all).