* [src] Build the .NET version of our product assemblies using a .NET 5 BCL.
We're not shipping the .NET product assemblies in any stable release, so we
can use a preview version of the .NET 5 BCL.
Also:
* Add all the nuget feeds we need to the top-level NuGet.config, even for .NET
5/6, there shouldn't be any conflicts with stable feeds since we use exact
version numbers.
* Generate a top-level global5.json which is copied to every directory that
needs a .NET 5 global.json (overriding the .NET 3.1 global.json in the root
directory).
* Use the expected dotnet binary during our local build.
* [tests] Fix the bgen tests to use .NET 5.
* [xharness] Set the current directory to the project directory when running .NET tests.
This way we end up using the dotnet version that's configured in global.json for the tests.
This makes us only put packages in one directory (saves disk space and time),
and it also makes project files in multiple solutions work properly
(mtouch.csproj is in tests/tests.sln and tests/mtouch/mtouch.sln).
Rolf Bjarne Kvinge
Waleed Chaudhry