Dynamic allocation for file descriptors used in ./build-host.sh

@klausenbusk would you consider it OK like that to just to use system-defined fd? It would close #135 (closed) (that is already closed as not an arch-boxes bug)

Edit: Visual studio code had an issue with file descriptors already used by the integrated terminal. This commit asks bash to dynamically assign a "free" file descriptor to be used in the expect function.