Add System.Formats.Nrbf.dll to SignList.xml

The package-reference bumps in this branch (Microsoft.Build 17.13.26 ->
17.14.28, System.Text.Json 8.0.5 -> 9.0.16, System.Reflection.MetadataLoadContext
8.0.0 -> 9.0.16) brought System.Formats.Nrbf.dll into each SDK NuGet's
tools/msbuild/net10.0/ folder. The signing step fails because the file isn't
listed in SignList.xml. It's a Microsoft-signed runtime assembly, so add it to
the <Skip> group alongside its sibling System.* tools entries.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dalexsoto

dotnet/Workloads/SignList.xml

@@ -48,6 +48,7 @@
     <Skip Include="System.Diagnostics.EventLog.dll" />
     <Skip Include="System.Diagnostics.EventLog.Messages.dll" />
     <Skip Include="System.Drawing.Common.dll" />
+    <Skip Include="System.Formats.Nrbf.dll" />
     <Skip Include="System.Reactive.dll" />
     <Skip Include="System.Resources.Extensions.dll" />
     <Skip Include="System.Security.Cryptography.Pkcs.dll" />