generateUnixyProxyCode() properly supporting Cygwin & Git Bash

Based on Composer's proxy: https://github.com/composer/windows-setup/blob/master/src/shims/composer

Previous `generateUnixyProxyCode()`'s version was generating unixy-proxy file which was causing problems:

```
$ phpunit
Could not open input file: /cygdrive/c/Users/<user>/AppData/Roaming/Composer/vendor/phpunit/phpunit/phpunit
```
please keep ``#!/usr/bin/env sh`` for the shebang rather than ``#!/bin/sh``