Enable multithreaded execution in NuGet static-graph restore

[rainersigwald]

In static graph restore, NuGet.Build.Tasks.Console.exe builds an MSBuild static graph (in parallel) and then executes targets on each project to determine ProjectReferences, PackageReferences, and so on that NuGet cares about for each project:

https://github.com/NuGet/NuGet.Client/blob/b83566ec2369c4e9fd07e6f95d734dfe370a1e66/src/NuGet.Core/NuGet.Build.Tasks.Console/MSBuildStaticGraphRestore.cs#L929-L947

Right now that target execution doesn't opt into multithreaded MSBuild, but enabling that should a) not use very many tasks (so easily enlightened) and b) benefit a lot from parallelism. We should enable multithreaded there.

[jankratochvilcz]

Status update from a recent multithreaded MSBuild sync:

Deferred for now. The team prefers to first prove MT's broader value before adding optimization asks to the NuGet team's plate. Revisit once MT is enabled by default in our pipelines and the perf story is established. Moving the sprint assignment to 2026 June II to reflect this.