Strange race condition on Windows

[vanyle]

I will, but it really feels wrong... This is Mac, not Nix / Arch. Non technical 
users trying out Nim should not be expected to compile their compiler. I know 
it's just `koch boot -d:release`, but like for a first time user (maybe coming 
from python), this is daunting. I really like the idea of detecting the CPU 
architecture at compile time or at least providing an ARM version of Nim 
through choosenim.