Extend macOS obuilder to support multi-user

Currently obuilder on macOS only supports a single user account, limiting the throughput on the macOS workers.
Extending macOS obuilder to support multiple users should improve throughput and resource usage.

Things to consider:

• homebrew assumes a single user install, extend the File system redirection to handle multiple users
• macOS base images might need to be relocatable so they can be shared between users
• opam root also needs to be sharable

Mark tried putting OPAMROOT on an obuilderfs mounted file system, unfortunately we see issues with opam installing software such as opam install dune and in some cases even opam init. He tested the macOS file system using https://github.com/pjd/pjdfstest and compared the results to a fuse-mounted file system. macOS directly failed 82 of the 8720 tests. When mounted through obuilderfs/macFuse approximately 3500 tests failed. https://ocamllabs.slack.com/archives/D02LTUAT74K/p1665657002246979

As macFuse doesn’t seem to be good enough other alternatives need to be investigated.

Start by reviewing the obuilderfs/macFuse filesystem behavior and reproducing the pjdfstest comparison described in the issue. Investigate alternatives that can support shared Homebrew installs, relocatable macOS base images, and a sharable OPAMROOT; done means macOS obuilder can serve multiple users while maintaining reliable filesystem and package installation behavior.