dotnetup: How to handle runtime installations if we override DOTNET_ROOT

If you use dotnetup to install the .NET SDK and then later run an app (or the app tries to run at starltup), you may get an error like this:

This is because dotnetup has set the DOTNET_ROOT path to your user-local dotnet folder which doesn't have the runtime that the app needs installed. The installer the error message links to is probably already installed, and even if it's not installing it wouldn't help.

We should improve the experience here.

Eventually the host (or whatever is showing the message) should probably detect that DOTNET_ROOT is set to a user location and offer to install the correct runtime via dotnetup.

Shorter-term solutions could include detecting DOTNET_ROOT and updating the error message, as well as installing existing admin runtimes into the user-local DOTNET_ROOT when dotnetup is first used.