Windows confirmation prompt not disabled with -y

[ctaggart]

rustup-init.exe -y doesn't suppress all the confirmation prompts on Windows with 1.0.0.
#912 (comment)

[brson]

From the linked issue

C:\WINDOWS\system32>"C:\Users\c\AppData\Local\Temp\chocolatey\rustup\1.0.0-beta2\rustup-init.exe" -y

Rust Visual C++ prerequisites

Rust requires the Microsoft C++ build tools for Visual Studio 2013 or later,
but they don't seem to be installed.

The easiest way to acquire the build tools is by installing Microsoft Visual
C++ Build Tools 2015 which provides just the Visual C++ build tools:

  http://landinghub.visualstudio.com/visual-cpp-build-tools

Alternately, you can install Visual Studio 2015 or Visual Studio 2013 and
during install select the "C++ tools":

  https://www.visualstudio.com/downloads/

Install the C++ build tools before proceeding.

If you will be targetting the GNU ABI or otherwise know what you are doing then
it is fine to continue installation without the build tools, but otherwise,
install the C++ build tools before proceeding.

Continue? (Y/n)

I'm not sure why this would happen. The test suite runs mostly with -y without providing input. Needs some investigation.

[pxdeu]

The test suite ignores the check because the environment variable 'RUSTUP_INIT_SKIP_MSVC_CHECK' is set to 'yes'.
no_prompt was simple not being checked.

This is my first pull request/contribution (#1124) ever. I hope everything is fine.