Add capability to supply env vars to gitlab-pages

To support continuous profiling in gitlab-pages (gitlab-com/gl-infra/scalability#334 (closed)) we need to supply an environment variable (GITLAB_CONTINUOUS_PROFILING). Before this MR, gitlab-pages had one env var hard-coded into the sv run file. This MR converts to using chpst's -e option as we do for many of our other services, and permits the configuring of additional env vars from gitlab.rb (as we do for e.g. gitaly and praefect).