[FR] use system installed unxip

[andrewcrook]

you can now tap and install uxip via Homebrew I think Xcode’s should try to use the installed version before downloading another.

[juanjonol]

I was thinking about making a Pull Request for this, but unxip's latest changes have just being added on #209 and I'm not sure unbundling unxip has much value (I literally haven't seen a .xip used for anything besides Xcode distribution).

Anyway, if someone decides to do this, xcodes' Homebrew formula should also be updated (as suggested for aria2 on https://github.com/RobotsAndPencils/homebrew-made/pull/5).