Disable dotnet/runtime 5.0 targeting pack build

[dagood]

We need to make sure the 5.0.0 targeting packs don't build and are generated in SBRP, if dotnet/runtime#1622 isn't completed in 5.0. The work to disable it might already be done but I'm not sure, filing this as a tracking issue for source-build functionality.

I wasn't able to find anything doing this in the current master branch, any ideas @crummel @dseefeld, or is this something that we know isn't done based on the current 5.0 WIP builds?

On the runtime side, @NikolaMilosavljevic could implement this disable in dotnet/runtime, or @jkoritzinsky may get the new sharedfx tooling in and obviate all of this.

(Related: dotnet/runtime#32982, where in triage I thought we had already patched this.)

[dagood]

We have Microsoft.NETCore.App.Ref,5.0.0 in the prebuilt list, which is pretty good evidence we have done this for the 5.0.0 GA build.