Switch macOS multi-arch to x86_64+arm64

The previous CI attemtped to use Homebrew for builds, but
unfortunately Homebrew has dropped support for universal packages aka
multiarch (fat) binaries. This means that in order to build an arm64 +
x86_64 package, macports has to be used instead of homebrew.

Unlike Homebrew, MacPorts is not installed by default on the GitHub
Actions runners, so we need to install it ourselves. This means
managing our own instance of the cache, which itself produces
challenges as the `gtar` binary run by the action doesn't have enough
permissions to restore the MacPorts checkout. So we have to shim gtar
with a sudo wrapper.

With this commit, we produce a Mosh package that works on macOS 11.0
and newer, on both arm64 and x86_64 architectures. The protobuf
library is statically linked, but all other libraries are provided by
the system.