Fix `dotnet run` to set `DOTNET_ROOT`.
Fix `dotnet run` to set `DOTNET_ROOT`.

This commit causes `dotnet run` to set `DOTNET_ROOT` to the muxer's directory
if not already set.

This enables `dotnet run` to work with an apphost when the runtime is not
installed to a default location.

Fixes #10196.

livarcocc/cli-1,dasMulli/cli,livarcocc/cli-1,johnbeisner/cli,livarcocc/cli-1,dasMulli/cli,johnbeisner/cli,dasMulli/cli,johnbeisner/cli