Use targets from the .NET SDK

[Forgind]

Fixes the MSBuild side of #7832

Specifically, the plan is that Roslyn will restructure their nupkg to include framework bits in a standard location. The SDK will then xcopy those somewhere then set this path. MSBuild will see that path (if it's set) and know to use that specific version of the roslyn targets to avoid version mismatches.

There isn't any required ordering between this part and the other two, and since it should have ~0 impact without the other two parts, I think it's fine to put it in now.

Context

Changes Made

Testing

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