Fix cygwin NoneType error if POSIX path in dest

[danyeaw]

Closes #1962. Corrects an AttributeError for a regex match not found if the cygwin path is already in POSIX format.

I am not able to reproduce the issue in #1962, but this PR guards against trying modify the virtualenv path if the regex doesn't return a match.

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• ran the linter to address style issues (tox -e fix_lint)
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• ensured there are test(s) validating the fix
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[codecov]

Codecov Report

Merging #1964 into main will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1964      +/-   ##
==========================================
- Coverage   93.93%   93.91%   -0.03%     
==========================================
  Files          86       86              
  Lines        4237     4239       +2     
==========================================
+ Hits         3980     3981       +1     
- Misses        257      258       +1     

Flag	Coverage Δ
#tests	93.91% <100.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
src/virtualenv/activation/via_template.py	100.00% <100.00%> (ø)
src/virtualenv/seed/embed/base_embed.py	96.22% <0.00%> (-1.89%)	⬇️

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[gaborbernat]