Generate prebuilt report and include it in the intermediate nupkg (arcade-powered source-build)

[dagood]

Add Microsoft.DotNet.SourceBuild.Tasks. This is separate from Microsoft.DotNet.Arcade.Sdk because the dependencies it takes on NuGet libraries are in danger of conflicting with what's loaded by Arcade already. See #6014. The tasks in this new package let us track prebuilts, for dotnet/source-build#1715.

To use the tasks in Microsoft.DotNet.SourceBuild.Tasks, we restore that package in Tools.proj if we're running source-build, and then use it from a new build step AfterSourceBuild.proj. (Since we're relying on Execute restoring Tools.proj, we now need to add a build step rather than doing everything in BeforeTargets=Execute.) AfterSourceBuild.proj only runs if '$(ArcadeBuildFromSource)' == 'true', so I believe these changes will have no effect on existing infra.

I also moved the intermediate nupkg generation from a BeforeTargets="Execute" target to AfterSourceBuild.proj, so I could include the prebuilt report generated in AfterSourceBuild.proj in the intermediate nupkg. This is for dotnet/source-build#1725.

I included a test project for the new tasks. Currently there's only a tiny test to justify the project's existence. The tasks are simply copied from dotnet/source-build, so there isn't any significant test coverage yet. I want to make sure we have a test project ready so we don't have a barrier to entry to adding regression tests and such in the future.