dotnet msbuild fails to restore from private Azure feed

### Steps to reproduce

1. Install dotnet sdk version 3.0.100-preview5-011568
2. Do `dotnet msbuild ` where has a reference stored in an internal Azure feed

Project file
```xml

```

Package source in nuget.config
```xml

```

(Ping me for the token)

When using dotnet version 2.1 the Arcade SDK is restored correctly as seen in `dotnet msbuild 2.1` of https://dev.azure.com/dnceng/internal/_build/results?buildId=219211.

dotnet version 2.1 and 3.0 work fine when doing `dotnet restore ` as seen in `CmdLine` of https://dev.azure.com/dnceng/internal/_build/results?buildId=217065 and https://dev.azure.com/dnceng/internal/_build/results?buildId=219105

I don't really know if the issue is in msbuild, nuget or CredProvider.

/fyi: @rainersigwald @zarenner