Fix fetch-configlet bash script for windows
Fix fetch-configlet bash script for windows

Sync with upstream https://github.com/exercism/configlet/pull/156

* This correctly sets the OS and EXTension when on windows, even when in bash
(or a different bourne-shell).

* Additionally this uses unzip if the extension is zip, as tar will correctly say the
zip is not a tar. `unzip` does not allow for piping, so a temporary file is created.

exercism/xkotlin,exercism/xkotlin