Picking a default windows toolchain by detecting msvc is unpredectable

[brson]

On Windows, rustup will detect whether MSVC exists, and if so will install the MSVC toolchain, otherwise GNU. That you get different software based on what happens to be preinstalled in your environment is unpredictable. As an alternative, we might instead do the detection, but always install the msvc toolchain, printing a warning with further instructions if you don't have MSVC.

cc rust-lang/prev.rust-lang.org#341 (comment) @Diggsey @vadimcn

[retep998]

We could also ask the user what they want to do.

[brson]

@retep998 The installer already surfaces an option to change the toolchain. It's fairly prominent, but it's not a direct question.

[Diggsey]

I think we always default to msvc now.