The actual fix here is for a compile error which started appearing as an
error due to `unistd.h` include being included implicitly from
`boost::signals`, which FastSignals does not do anymore.
```
Error:
/Users/runner/work/FreeCAD/FreeCAD/src/Gui/Dialogs/DlgVersionMigrator.cpp:366:5:
error: use of undeclared identifier '_exit'
```