Add build libunity editor script - #1

Missing Unity License File and no Serial was found. If this is a personal license, make sure to follow the activation steps and set the UNITY_LICENSE GitHub secret or enter a Unity serial number inside the UNITY_SERIAL GitHub secret.

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