Inspired by https://github.com/dotnet/core/issues/6419.

This message is reasonable for Windows scenarios, but since there are no .NET Framework Developer or Targeting Packs for Linux or macOS, we shouldn't mention them in that circumstance.

error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks

Not sure whether we should explicitly mention Microsoft.NETFramework.ReferenceAssemblies. The SDK should provide it for you so if you hit this on *nix it's likely an SDK bug.