ci: pin third-party GitHub Actions to commit SHAs - #171

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff, golangci/golangci-lint-action@3a919529898de77ec3da873e3063ca4b10e7f5cc. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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