Don't use CI_PROJECT_DIR variable

All environment variables that start with CI_* are blacklisted as detailed here: gitlab-org/gitlab#37938 (comment 254231503)

CI_PROJECT_DIR was used to attempt to support compilation of Helm templates, but as full support for that didn't make it into the initial release, this MR undoes that work.

And here's the follow-up issue to add proper Helm template compilation support.