west: spdx: Fix --init for Windows builds

Currently, west spdx --init uses os.mknod to create an empty file
to enable the Cmake file-based API system. As reported in #39311,
Python on Windows does not implement os.mknod.

This commit switches to using open()/close() instead of os.mknod()
to address this issue.

Signed-off-by: Steve Winslow <steve@swinslow.net>