fix: avoid deprecated os.path.commonprefix() in RelativePath - #365

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: seanmiddleditch/gha-setup-ninja@3b1f8f94a2f8254bd26914c4ab9474d4f0015f67. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/