Add new package source for DotNet-msbuild-Trusted

Author: marcpopMSFT

NuGet.config

@@ -30,6 +30,7 @@
     <add key="darc-int-dotnet-aspnetcore-d3aba8f" value="https://pkgs.dev.azure.com/dnceng/internal/_packaging/darc-int-dotnet-aspnetcore-d3aba8fe/nuget/v3/index.json" />
     <!--  End: Package sources from dotnet-aspnetcore -->
     <!--  Begin: Package sources from DotNet-msbuild-Trusted -->
+    <add key="darc-pub-DotNet-msbuild-Trusted-2681ee4" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-DotNet-msbuild-Trusted-2681ee4b/nuget/v3/index.json" />
     <!--  End: Package sources from DotNet-msbuild-Trusted -->
     <!--  Begin: Package sources from dotnet-roslyn-analyzers -->
     <!--  End: Package sources from dotnet-roslyn-analyzers -->