Do not condition RAR cache on BuildingProject=false (#2812)
Do not condition RAR cache on BuildingProject=false (#2812)

VS design time builds have BuildingProject=false. This makes them not use RAR caching.
By enabling DTB RAR caching, DTB RAR time gets cut down in half on warm caches, and remains mostly unchanged on cold caches.

Times (measured design time builds via the Project System Tools extension 3 times and averaged, on an mvc project):
- no cache: 210ms
- cold cache: 211ms
- warm cache: 108ms

#closes dotnet/project-system#3022
rainersigwald/msbuild,chipitsine/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,mono/msbuild,chipitsine/msbuild,Microsoft/msbuild,mono/msbuild,cdmihai/msbuild,cdmihai/msbuild,cdmihai/msbuild,Microsoft/msbuild,cdmihai/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,Microsoft/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,chipitsine/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,mono/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,mono/msbuild,chipitsine/msbuild