Use PHP_WINDOWS_VERSION_BUILD to check for Windows in test tasks

Symfony Process also does it like that (which is used to execute the command).

`!empty($_SERVER['windir'])` did not work for me.
Tested it with normal CMD and Git Bash.

(This also fixes the bug introduced in https://github.com/Codegyre/Robo/commit/b53fe4cf44f784c9bf6d8a88d8b631784e8e0d2b#diff-f2bf6c27b538bfc61de4fb46d65d0f9dR41)