Introduce msbuild/toolset selection log switch for arcade

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

Every once in a while, we need to figure out why arcade is choosing a specific dotnet, arcade, msbuild, etc. The logic for the selection has grown organically over the years to support many different scenarios, and debugging why a specific engine was chosen can be pretty hard.

Introduce a switch or env-var to log the process. This will save a lot of time over the long haul.