Fix intermittent failure in DllImportGenerator unit tests when resolving assemblies

[elinor-fung]

Since the tests run in parallel, we could end up clearing out the variable for redirecting the package cache while it was still needed by another test.

Fixes #60706

[steveisok]

On the mono windows x64 leg (staging pipeline), we're getting a whole bunch of these in the DllImportGenerator tests:

NuGet.Protocol.Core.Types.FatalProtocolException : Unable to load the service index for source https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json.
---- System.Net.Http.HttpRequestException : Unknown error (0xffffffff) (pkgs.dev.azure.com:443)
-------- System.Net.Sockets.SocketException : Unknown error (0xffffffff)

[elinor-fung]

I suspect that is a manifestation of #53281. We can disable these unit tests for mono on Windows.