Also add a very simple net5 test app (which doesn't build yet).
Current (expected) build result:
> xamarin-macios/tests/dotnet/MySingleView $ dotnet build
Microsoft (R) Build Engine version 16.7.0-preview-20258-02+26f6d1d87 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.
Determining projects to restore...
xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj : error NU1102: Unable to find package Microsoft.NETCore.App.Runtime.ios-x64 with version (= 5.0.0-preview.6.20264.1)
xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj : error NU1102: - Found 1 version(s) in nuget.org [ Nearest version: 5.0.0-preview.4.20251.6 ]
xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj : error NU1102: - Found 1 version(s) in Nuget Official [ Nearest version: 5.0.0-preview.4.20251.6 ]
xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj : error NU1102: - Found 0 version(s) in local-dotnet-feed
xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj : error NU1102: - Found 0 version(s) in Dotnet arcade
Failed to restore xamarin-macios/tests/dotnet/MySingleView/MySingleView.csproj (in 951 ms).
This fails because the .NET build logic isn't being told that it should look
for the mono runtime packs instead of the .NET Core runtime packs.