Build universal binaries for macOS

[Dan Callahan]

It'd be nice if our darwin binaries contained both ARM and Intel code. Avoids a potential class of user error.

Looks like GoReleaser supports universal binaries: https://goreleaser.com/customization/universalbinaries/

Alternatively, can manually call lipo ./subo-arm ./subo-intel -create -output ./subo-universal

Related Apple docs: https://developer.apple.com/documentation/apple-silicon/building-a-universal-macos-binary