aspnetcore is missing artifacts from getting signed via in-built signing in the VMR

[ViktorHofer]

https://github.com/dotnet/aspnetcore/blob/e78c1a3ed28b771954596c293752faa9293fe6ee/eng/Signing.props#L19-L26

(Unix-) Assets that are getting published in Publishing.props are missing from that list. Ideally aspnetcore would do the following:

1. Stop resetting Arcade's defaults: https://github.com/dotnet/aspnetcore/blob/e78c1a3ed28b771954596c293752faa9293fe6ee/eng/Signing.props#L5-L10
2. Using the Artifact extension point in Signing.props so that items are in sync between Signing and Publishing.

cc @ellahathaway @jkoritzinsky @mmitche