Make ALL `dotnet sdk` branches use `dotnetup`

The best way to get large scale dogfooding and testing of the product once we think it works consistently enough is to put it in the `main` branch of the SDK. This will replace the install script usage, just like how release/dnup today uses dotnetup to install the runtimes.

# Pending
It will also use dotneutp to install the daily or internal build SDK once https://github.com/dotnet/sdk/pull/54061 is complete

We will also need a GH release to download dotnetup from instead of building dotnetup, since the sdk repo `main` will still not include `dotnetup`.

Once this is in and we have confidence for >= ~1 week, we should expand it to the other `release/*` branches of the .NET SDK.