zephyr-env.sh: only reset scripts_path on Windows

The pipeline that converts scripts_path from "C:\" style to "/c/"
style is only needed on Windows. Further, on Linux, it prepends an
extra "/" to what gets added to PATH, since the pwd output already has
a leading "/".

Fix that by only making the change when we're building on Windows.

Change-Id: Ied7e4491f171d82e8dee4c3a4dfdcf64adf46efd
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>