cpp-gsl: update checksum and cmake build syntax

[chenrui333]

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• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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upstream has re-tagged the release per:

• Release version not changed when 4.1.0 was tagged microsoft/GSL#1161
• GSL v4.1.0 microsoft/GSL#1163

[github-actions]

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