Fix test suite failing if run under system Python

[gaborbernat]

Resolves #1882

[codecov]

Codecov Report

Merging #1888 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1888      +/-   ##
==========================================
+ Coverage   93.86%   93.96%   +0.09%     
==========================================
  Files          86       86              
  Lines        4175     4175              
==========================================
+ Hits         3919     3923       +4     
+ Misses        256      252       -4     

Flag	Coverage Δ
#tests	93.96% <ø> (+0.09%)	⬆️

Impacted Files	Coverage Δ
src/virtualenv/create/creator.py	96.06% <0.00%> (+0.78%)	⬆️
src/virtualenv/util/subprocess/_win_subprocess.py	95.08% <0.00%> (+1.63%)	⬆️
src/virtualenv/util/path/_sync.py	93.10% <0.00%> (+3.44%)	⬆️

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