I think you’re both hitting the same problem - we needed some quotes around the appx path to signtool. Try again with commit 9470688.
@Keresh
Yes, a build for Lumia 950 would require ARM which is not currently available. We don’t have anything to announce concerning future plans for this fork, but please do keep providing feedback on what you might be interested in. And if you decide to add ARM support yourself do let us know how it goes.