The cinematic sky extension was removed from the demo project 25 days ago :D Check the latest demo, the extension now ships with its own skybox which you set in the VrHeadAnchor behavior (and it doesn't work the same as GDevelop skybox - check the demo) because of a bug in GDevelop where it strips background when rendering for VR.
More info here: https://avram.itch.io/webxr-gdevelop-extension/devlog/1593430/update-crouch-fix-...