vm/windows: Allow to specify a different arch.

So far, we determined this based on stdenv.is64bit, but there are cases
where you want to run/build a 32bit program on a 64 bit Windows.

This is now possible, by passing windowsImage.arch = "i686" | "x86_64"
to runInWindowsVM. Based an what was passed, the corresponding Cygwin
packages and setup.exe are bootstrapped.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>