Add as-console-user command

- Support MDM/Munki/Jamf runs where the parent process is root but
  Homebrew must execute as the logged-in macOS console user.
- Keep the nested operation constrained to `HOMEBREW_BREW_FILE` so this
  helper cannot become a general-purpose root command launcher.
- Share `macos_user.sh` with installer scripts so no-user and package
  plist fallback behaviour cannot drift between install paths.
- Stage `macos_user.sh` for `pkgbuild` because `preinstall` needs the
  resolver before the Homebrew payload has been installed.