SetupNugetSources.sh writes invalid XML comments when re-enabling commented-out darc-int feeds

[ellahathaway]

See dotnet/source-build#5349

The script eng/common/SetupNugetSources.sh writes invalid XML comments like:

<!-- <!-- Reenabled for build : <source> --> -->

when re-enabling disabled darc-int feeds in NuGet.Config that are already commented out. This is not valid XML and causes NuGet errors.

The script should be updated to only write single-layer XML comments (e.g., <!-- Reenabled for build : <source> -->), and avoid double hyphens or trailing hyphens in comments.

[mmitche]

The script should just not write out any comments for this, I think.