`dotnetUp` should detect admin installs on Unix

[nagilson]

#51101 tracks what to do with the admin installs we've detected, but we currently only detect windows installs. We need to implement the logic to track admin installs on mac/linux - a great starting point is the etc/install_location(s): https://github.com/dotnet/designs/blob/main/accepted/2020/install-locations.md