Don't overbuild source generator when root building for net48

[ViktorHofer]

Fixes #105364

Just noticed that source generators are now built twice. Regressed with e2a3511

Adding this blurb as in oob-all, oob-ref and oob-src.proj fixes that.

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[ericstj]

Won't this still overbuild in the case that a generator doesn't multi-target? What was causing the overbuild to happen? I would have expected any duplicates to hit the MSBuild project cache - were global properties different?

[ViktorHofer]

Different global properties, yes. More precisely, the BuildTargetFramework property set.

This solution works for both multi targeting and non multi-targeting builds and is just following what we already do in the other oob-*.projs. I simpy forgot to add that part.

See https://github.com/dotnet/runtime/blob/main/eng%2FTraversalSdk.AfterTargets.targets

[ViktorHofer]

/ba-g CI pipeline is stuck (GH thinks it still runs but it already canceled on AzDO side)

[ViktorHofer]

@ericstj I'm going ahead merging this as the failure is observable in CI already. I'm happy to react to any feedback in a follow-up.

[ericstj]

I see - I am better understanding this change now. Before I was worried we'd force a build with the TargetFramework value of BuildTargetFramework, but I see that instead you're evaluating the child projects for that framework.

I wonder if this should actually be the default for all Traversal projects when BuildTargetFramework is set.