Cap the libxc build parallelism at 4 - #337

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/