[net9.0] Bring back missing change from #20790 to SignList (#21323)

This change is missing from the last revert
25509d7917
that happened on https://github.com/xamarin/xamarin-macios/pull/21264

dotnet/Workloads/SignList.xml

@@ -122,7 +122,7 @@
     <FirstParty Include="Broker.exe" />
     <FirstParty Include="Broker.resources.dll" />
     <!-- Build.zip -->
-    <FirstParty Include="Build.exe" />
+    <FirstParty Include="Build.dll" />
     <FirstParty Include="Microsoft.Build*.dll" />
     <FirstParty Include="Microsoft.NET.StringTools.dll" />
     <FirstParty Include="System.IO.Abstractions.dll" />