Fixes variable export for Windows.

The solution presented in #1870 has been implemented only partially.

The fix in commit https://github.com/laravel/laravel/commit/678b92ef85ce0258eb9a6ac57bb26fca9480e430 will not work.

This Pull Request makes an additional correction so that ```LARAVEL_ENV``` can be used in Windows. 

Signed-off-by: aeberhardo <aeberhard@gmx.ch>