gitlab-ci.yml: Update CI_BUILD_REF_NAME

With GitLab 16.0 CI_BUILD_REF_NAME was replaced with CI_COMMIT_REF_NAME.
which might explain why we do not seem to install the documentation on
https://wayland.pages.freedesktop.org/weston/



This should makes that docs deplay stage run.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>