Without the desktop plug, launching the syncthing snap will not launch a browser window to the admin UI. Adding this one line will fix that. Tested here on my Ubuntu system with a build from tip of master.
This makes the environment variables easier to read/change.
Also expand the list so it's not inline, more readable that way.
The new architecture syntax for snapcraft allows specifying both the building architecture and the running architecture of the snap, so if we specify the build-on architecture as the host architecture and the run-on architecture as the target architecture, then snapcraft shouldn't need to install any cross-compilers, etc.
