debuild: Adjust the arm64 build to use Maurits Lamers' nwjs package.

This is a followup to !732 (merged) which further adjusts our debuild machinery so that arm64 packages can now be built successfully on the OBS. An exemplary Debian 10 arm64 build is available here (go to "Download package", choose "Debian" and then follow the instructions; or you can just grab the arm64.deb package right here: https://download.opensuse.org/repositories/home:/aggraef:/purr-data-git/Debian_10/arm64/purr-data_2.16.0+git4826+48ca6e4d-1_arm64.deb).

This build uses the only arm64 build of nw.js that seems to be available anywhere right now, which is a build of nw.js 0.23.7 done some time ago by Maurits Lamers, available here: https://github.com/LeonardLaszlo/nw.js-armv7-binaries/releases/tag/v0.23.7

I haven't tested the resulting binary yet. But at least it builds on the OBS now. I invite anyone with an arm64 Debian 10 system to give it a go.