dotnet-svcutil.xmlserializer tool fails to load assemblies when targeting netcoreapp3.1

[StephenBonikowsky]

This is the same problem with the Microsoft.XmlSerializer.Generator as reported in dotnet/runtime#630

The tool currently runs on netstandard2.0 so if the application using the tool targets a newer framework such as netcoreapp3.1 it will fail to load the newer version of assemblies that have already been loaded by the tool.

[imcarolwang]

I've validated that this issue no longer reproduces in the private build created from the latest code (with PR #5321 merged) .