Make sure that NS2.0 source generators are built

97a51cc regressed the inclusion of source generators in the targeting pack as the `netstandard2.0` inner build was never chosen by the sfx-gen.proj traversal project. Noticed in the SDK consumption PR: dotnet/sdk#29406

Author: ViktorHofer

src/libraries/sfx-gen.proj

@@ -1,7 +1,9 @@
 <Project Sdk="Microsoft.Build.Traversal">
 
   <PropertyGroup>
-    <TargetFramework>$(NetCoreAppCurrent)</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
+    <!-- Filter ProjectReferences to build the best matching target framework only. -->
+    <FilterTraversalProjectReferences>true</FilterTraversalProjectReferences>
   </PropertyGroup>
 
   <!-- Reference all NetCoreAppCurrent shared framework generator projects. -->