Fix legacy OVRTX cloning of instanced geometry and unused variants - #7871

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Summary report available at: https://github.com/isaac-sim/IsaacLab/actions/runs/35284413935#summary-105413447489