Build improvements for preview packs

- [x] Disable TwbT dfhack init settings if DFHack is installed but TwbT is not
- [x] Omit TwbT from contents if it's not installed (note that this happens during extraction - maybe an up-front check is needed?)
- [ ] Automatically pull DFHack nightly if `--unstable` is passed and there's no stable DFHack for this version yet

There's probably more which I'll discover soon...