Obsoleted CI variables

Summary

CI variables that we are still using ($CI_BUILD_*) are deprecated.

Note: Starting with GitLab 9.0, we have deprecated the $CI_BUILD_* variables. You are strongly advised to use the new variables as we will remove the old ones in future GitLab releases.

Possible fixes

We need to use new names, see https://docs.gitlab.com/ee/ci/variables/

cc: @anselmstordeur