Only set GitLab Pages variable if GitLab Pages is enabled on that project
Problem to solve
Follow up to the comment on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24504#note_136193341
If GitLab Pages are disabled for the project, then GiaLab Pages related variables shouldn't be set during a build.
- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
This is a feature that was requested after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24504 was merged.
Figure out how to detect is GitLab Pages are enabled, and then only set the GitLab Pages environment variables.
What does success look like, and how can we measure that?
If GitLab Pages is enabled: environment variables
If GitLab Pages is disabled: environment variables
CI_PAGES_URL do not exist