Clear stash if system executable is missing

[gaborbernat]

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

[codecov]

Codecov Report

Merging #2050 (d77e96e) into main (39fdf95) will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #2050      +/-   ##
==========================================
- Coverage   93.68%   93.62%   -0.07%     
==========================================
  Files          88       88              
  Lines        4356     4360       +4     
==========================================
+ Hits         4081     4082       +1     
- Misses        275      278       +3     

Flag	Coverage Δ
tests	93.62% <50.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
src/virtualenv/discovery/cached_py_info.py	93.00% <50.00%> (-1.80%)	⬇️
src/virtualenv/util/path/_pathlib/__init__.py	93.93% <0.00%> (-3.04%)	⬇️

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[gaborbernat]

Released now via https://virtualenv.pypa.io/en/latest/changelog.html#v20-3-1-2021-01-13