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

## 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.