sandbox: fallback to tput for winsize

[Bo98]

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This should improve compatibility with some tests sensitive to the terminal size in the non-tty (CI) environment, such as oakc.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Code looks fine to me but can we get some more extensive comments in here as to:

• what is happening here
• why this is being done
• how this is being done

[Bo98]

Have a look at the latest commit and see if the comments make sense.

[MikeMcQuaid]

Looks great, thanks! When you can be bothered (in a follow-up PR) I'd love to see similar level of comments on the other surrounding code around this security feature. ❤️