Support the new snupkg format

src/Directory.Build.props

 <Project>
   <Import Project="..\Directory.Build.props" />
 
+  <PropertyGroup>
+    <IncludeSymbols>true</IncludeSymbols>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+  </PropertyGroup>
+
   <ItemGroup>
     <PackageReference Include="SourceLink.Create.CommandLine" Version="2.8.3" PrivateAssets="All" /> 
   </ItemGroup>