fix: make portable seek_and_modify_string_in_file (mac OsX)

[RedLeader962]

Description

Summary:

n2st::seek_and_modify_string_in_file was not portable. MacOsx implementation of sed does not have a --in-place flag. Refactor using -i

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Checklist:

Code related

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[RedLeader962]

🎉 This PR is included in version 2.1.3 🎉

The release is available on GitHub release

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