Upgrade CLI version and buildtools, and make build file changes.

Upgrade CLI version and buildtools, and make build file changes.

Unset CORE_ROOT in environment on Windows before doing restore so that dotnet CLI doesn't try to run using binaries from the test CORE_ROOT.

Restore the xunit wrapper projects in a batch rather than individually to save a lot of time.

Work around xunit.runner.msbuild not having compatible package layout by ignoring errors and adding import so that a future fix will work.

Hard-code C# language for generated wrapper projects: the $(Language) property wasn't set.

Also Change www.myget.org to dotnet.myget.org, as we switched to enterprise myget.

Add System.ObjectModel to tests\src\JIT\config\benchmark+serialize\project.json to manually upgrade it to a version that doesn't require lifting for runtime. This fixes System.ObjectModel.dll being missing from CORE_ROOT and causing the test to fail.

Switch to v3 nuget feeds and sync NuGet.Configs.