SBRP doesn't use live targeting packs

[ViktorHofer]

The following list of packages show up as prebuilts when removing the SBRP self-reference in SBRP:

Microsoft.NETCore.App.Ref.5.0.0
Microsoft.NETCore.App.Ref.6.0.0
Microsoft.NETCore.App.Ref.7.0.0
Microsoft.NETCore.App.Ref.8.0.0
Microsoft.NETCore.App.Ref.9.0.0
Microsoft.NETCore.Platforms.1.1.0
NETStandard.Library.2.0.3

This indicates that the corresponding live-built packages aren't used in the build later.

[MichaelSimons]

Fixed with dotnet/source-build-reference-packages#1192.