fix: install_unix.sh breaks with status code 2 in non-interactive environments

[dithmer]

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Proposed Change

The script "install_unix.sh" was not working in non-interactive shells, because there are tools used (stty and tput) for which a connected tty is mandatory. This is being fixed by a sh-compliant way of checking if the $- contains the i-flag.

Checklist

• [x] Changes are tested
• [x] CI has passed

[CLAassistant]

All committers have signed the CLA.

[Dylan-M]

I suggest closing this in favor of #1895

[BinaryFissionGames]

I'm going to close this since #1895 has been merged and should solve this problem with an optional flag. Thanks for taking the time to open this PR, it's very much appreciated!