Add `DOTNET_MULTILEVEL_LOOKUP` .NET environment variable

[IEvangelist]

We should come back to this new environment variable, and make sure that we add it to the .NET environment variables article.

Originally posted by @IEvangelist in #28965 (comment)

[IEvangelist]

Actually, I just realized this is already covered:

• https://docs.microsoft.com/dotnet/core/tools/dotnet-environment-variables#dotnet_multilevel_lookup