We should automatically use .net installed on the machine

See for instance https://github.com/microsoft/vscode-azurearmtools/issues/1081. Customer had dotnet installed and expected it to just work. Not sure there's really a reason why it shouldn't, is there? For instance, the extension could spawn a "dotnet --info" to determine which dotnet runtimes are currently installed and just use "dotnet" without a path if it's sufficient.