Update NuGet creds doc on new secret mount syntax

[Docker Engine 20.10](https://docs.docker.com/engine/release-notes/#20100) now [supports](https://github.com/moby/buildkit/pull/1534) the use of environment variables being used for secrets. This should be [documented](https://github.com/dotnet/dotnet-docker/blob/master/documentation/scenarios/nuget-credentials.md) as an option to use for handling NuGet credentials since environment variables are more likely to be used for these scenarios than files.