* [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.
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