Fix activator path in cygwin and msys2

[danyeaw]

Closes #1940. In cygwin and MSYS2, the path is in POSIX format. This PR converts the Windows path to POSIX format using a regex so that it is added to the bash activation script in the correct format.

Thanks for contributing, make sure you address all the checklists (for details on how see

development documentation)!

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[codecov]

Codecov Report

Merging #1952 into main will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1952      +/-   ##
==========================================
- Coverage   93.92%   93.91%   -0.02%     
==========================================
  Files          86       86              
  Lines        4228     4237       +9     
==========================================
+ Hits         3971     3979       +8     
- Misses        257      258       +1     

Flag	Coverage Δ
#tests	93.91% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

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%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4bcf20e...927aa92. Read the comment docs.

[danyeaw]

Anything else needed on this one?

[danyeaw]

Thanks @gaborbernat !