macOS: Create universal binaries (fat archives)

macOS supports bundling both the x86_64 and the arm64 binary into a single file using `lipo`. It would be nice to do this by default so that mac users don't have to think about their architecture when picking which file to download.

Start by locating the macOS artifact-building and download-packaging entry points in cargo-dist, then inspect how x86_64 and arm64 releases are produced. Use lipo to combine the two binaries by default, and verify that the published macOS download is a universal binary usable on both architectures.

Written by the indexing model from the issue text.