Changes in `NuGet.config` managing `darc-*` channels should not be considered meaningful

[premun]

Context

When we have stable versioning, the backflow updates packages but also has to add the stable feed. This is also uninteresting and should be considered meaningful.

Example

Example PR that is not meaningful: https://github.com/dotnet/dotnet/pull/3018/files

Goal

• Get the list of unique darc- feeds where the packages from the previous build are located in.
• Add those feed names to the list of strings we match against when detecting meaningful changes.