[sbrp] Targeting packs assemblies do not get the custom SBRP attribute

[NikolaMilosavljevic]

We have some build infra that is supposed to add custom SBRP attribute to targeting pack assemblies: https://github.com/dotnet/source-build-reference-packages/blob/432b22e3cdaf69c011358a44e61959769b2f607c/src/targetPacks/Directory.Build.props#L6

This infra does not work. Assemblies in targeting packs do not have the attribute defined here: https://github.com/dotnet/source-build-reference-packages/blob/432b22e3cdaf69c011358a44e61959769b2f607c/src/SourceBuildAssemblyMetdata.cs#L4

There is a similar infra for ref packs that works fine.