build: package the single-file header script for source builds - #43

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, actions/checkout@v4, hendrikmuhs/ccache-action@v1.2, mozilla-actions/sccache-action@v0.0.9. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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