Add arch-specific macOS builds

Currently, we bundle the Universal build of Ark in every macOS build of Positron: https://github.com/posit-dev/positron/blob/3d9663adbb27d76c76d6df54176c8e6c3a2067bc/extensions/positron-r/scripts/install-kernel.ts#L77-L79 For efficiency, we should bundle the architecture-specific build of Ark instead, and in order to that, we need to publish the architecture-specific builds as release artifacts.