[ci] fix shellcheck warnings in CI scripts #9100

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5 errors and 1 warning

unable to access 'https://gitlab.com/libeigen/eigen.git/': The requested URL returned error: 503

clone of 'https://gitlab.com/libeigen/eigen.git' into submodule path '/Users/runner/work/LightGBM/LightGBM/external_libs/eigen' failed

unable to access 'https://gitlab.com/libeigen/eigen.git/': The requested URL returned error: 503

clone of 'https://gitlab.com/libeigen/eigen.git' into submodule path '/Users/runner/work/LightGBM/LightGBM/external_libs/eigen' failed

The process '/usr/local/bin/git' failed with exit code 1

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.