[Windows][melodic] Fix _CUSTOM_PYTHONPATH_ENV on Windows build

[seanyen]

On Windows, even if the project calling generate_dynamic_reconfigure_options() doesn't have ${CATKIN_GLOBAL_PYTHON_DESTINATION}, it still needs to set _CUSTOM_PYTHONPATH_ENV to something like python, because Windows doesn't have shebang support and cannot run the <generated>.cfg directly.

(This change is verified in https://aka.ms/ros project.)

[seanyen]

To add more information: This problem manifests when nodelet_topic_tools builds, where it doesn't call catkin_python_setup() and doesn't have ${CATKIN_GLOBAL_PYTHON_DESTINATION}.

[seanyen]

@mikaelarguedas hope I am pinging the correct person. This is another Windows specific issue found in https://aka.ms/ros. I'd love any feedbacks and see if this can be merged to the upstream. Thanks!

[mjcarroll]

@seanyen-msft I'm the maintainer, I'll take a look. Thanks for the patch.

[seanyen]

Thanks!