CI - Smoketests fail if NuGet package is more recent than local one

[bfops]

Apparently our C# quickstart test will fail if we have published more recent package versions to nuget than are available locally. Apparently nuget will look at all sources to determine the latest package, even if some sources are not allowed for actually downloading the package in question.

We should fix this, maybe by having the dotnet add invocation use an explicit path, but it also shouldn't come up that frequently. Usually we bump versions in master before we publish to nuget.