Make `arcade` use `dotnetup`

After https://github.com/dotnet/sdk/issues/54257 this step, and after the SDK branches are all using dotnetup consistently without issue, we should push to make Arcade (and thus all of the repos in dotnet) use dotnetup to install the needed components for building. This will allow us to get more feedback, enable signing validation on the developer bits, and hopefully improve the workflow for getting started / building. (Better concurrency, etc - admittedly though more download IO due to downloading dotnetup... it's possible we could build dotnetup into /eng/ or arcade to prevent this.)