Explicit disabling of batch file delayed expansion

Windows environment hooks are a nesting of batch files. Changes in the way catkin sets up environment hooks will likely require the use of delayed expansion in upper level batch scripts that call this script. Delayed expansion settings are inherited through the chain of execution when batch files call each other.

Although it was commented that delayed expansion should be disabled for the period of echo'ing python code, this patch explicitly disables delayed expansion for that portion of the file.

See this pull request in win_ros for further context: [win_ros pull request #44](https://github.com/ros-windows/win_ros/pull/44)