Don't return available destinations when retreating aircraft if the runway is not operational

[geofffranks]

Short circuits selecting control points to retreat aircraft to when the control point being captured has a damaged runway.

Fixes #3434.

I'm not sure how to test this locally though, so I have no idea if it works.

[zhexu14]

Thanks for raising the PR. The Wiki has a developer's guide which walks through how to set up a development environment. I've also enabled the build, so as an alternative you can just download the built package from the build artefacts.

https://github.com/dcs-liberation/dcs_liberation/wiki/Developer's-Guide

[geofffranks]

Thank you! I updated to make sure Black is passing via the pre-commit hook (though also i'm seeing additional changes requested for black when run through VS Code.. not sure if that's a config difference, or if i should commit those additional changes). I've now tested this out out locally and it works!