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