Fix Signing validation failing to restore due to Package source mapping (#380)

Author: joperezr

NuGet.config

@@ -32,6 +32,7 @@
       <package pattern="Microsoft.DotNet.Maestro.Tasks" />
       <package pattern="Microsoft.DotNet.RemoteExecutor" />
       <package pattern="Microsoft.DotNet.SignTool" />
+      <package pattern="Microsoft.DotNet.SignCheck" />
       <package pattern="Microsoft.DotNet.Tar" />
       <package pattern="Microsoft.DotNet.XliffTasks" />
       <package pattern="Microsoft.DotNet.XUnitExtensions" />