API: Allow passing on the environment variable as an argument

[gaborbernat]

Defaults to os.environ.

Signed-off-by: Bernát Gábor bgabor8@bloomberg.net

[codecov]

Codecov Report

Merging #2054 (b01d6ad) into main (7185e57) will decrease coverage by 3.90%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##             main    #2054      +/-   ##
==========================================
- Coverage   93.64%   89.74%   -3.91%     
==========================================
  Files          88       88              
  Lines        4360     4378      +18     
==========================================
- Hits         4083     3929     -154     
- Misses        277      449     +172     

Flag	Coverage Δ
tests	89.74% <96.87%> (-3.91%)	⬇️

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Impacted Files	Coverage Δ
...virtualenv/seed/embed/via_app_data/via_app_data.py	82.17% <ø> (ø)
src/virtualenv/util/subprocess/_win_subprocess.py	0.00% <0.00%> (-95.32%)	⬇️
src/virtualenv/run/__init__.py	95.94% <90.00%> (+0.17%)	⬆️
src/virtualenv/discovery/builtin.py	81.88% <94.11%> (+0.43%)	⬆️
src/virtualenv/__main__.py	100.00% <100.00%> (ø)
src/virtualenv/app_data/__init__.py	94.11% <100.00%> (+0.17%)	⬆️
src/virtualenv/config/cli/parser.py	95.78% <100.00%> (+0.18%)	⬆️
src/virtualenv/config/env_var.py	100.00% <100.00%> (ø)
src/virtualenv/config/ini.py	89.83% <100.00%> (+0.17%)	⬆️
src/virtualenv/create/creator.py	95.41% <100.00%> (-0.74%)	⬇️
... and 17 more

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