Support Universal Apps

Currently the feed URL is constructed from [`process.arch`](https://github.com/electron/update-electron-app/blob/master/index.js#L39C68-L39C68) which doesn't offer any insight into how the app that's calling it is built. I'm not sure it's possible for the running code to determine how the app has been built.

My sense is that having a matrix of possibilities (darwin + x64, darwin + arm64, darwin + universal) to check would be worthwhile. Obviously this will also require some work in https://github.com/electron/update.electronjs.org to support it; I've filed https://github.com/electron/update.electronjs.org/issues/140 to track that.