passenv PROCESSOR_ARCHITECTURE by default on windows

[asottile]

Resolves #740

[codecov]

Codecov Report

Merging #742 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #742      +/-   ##
==========================================
+ Coverage   94.67%   94.67%   +<.01%     
==========================================
  Files          11       11              
  Lines        2383     2384       +1     
==========================================
+ Hits         2256     2257       +1     
  Misses        127      127

Impacted Files	Coverage Δ
tox/config.py	97.75% <100%> (ø)	⬆️

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