CI: add xrOS pipeline on the new xcode15 runner

This adds an additional xrOS / visionOS pipeline. It runs on a newly spawned runner hosted at scaleway that deploys Xcode 15.4 on macOS 14.5 to do the builds. It is the idea (if this proves to be stable) to transition the other appleOS pipelines for the 4.0 branch to this runner while keeping the existing configuration for the duration of the v3 lifetime with their respective builds.