gh-151042: venv: Pass VIRTUAL_ENV through cygpath inside fish on Windows

[LuNoX]

gh-151042: Fixed venv exporting mixed path styles to PATH on Windows inside fish (via Cygwin, MinGW or MSYS2)

This PR mirrors the changes from gh-103325, gh-125399 and gh-112508 to the fish activation script.

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[LuNoX]

@vsajip@FFY00 the equivalent PR was merged in both uv and virtualenv if you would like to compare for review :)

[vsajip]

This patch and the virtualenv patch have

if string match -qr 'CYGWIN|MSYS|MINGW' (uname)

whereas the uv patch has

if string match -qr 'CYGWIN|MSYS|MINGW' (uname); and command -s cygpath >/dev/null

Isn't the more stringent check needed? Under CYGWIN one might expect cygpath to be available, but is it guaranteed to be present under MSYS/MINGW?

[LuNoX]

This patch and the virtualenv patch have

if string match -qr 'CYGWIN|MSYS|MINGW' (uname)

whereas the uv patch has

if string match -qr 'CYGWIN|MSYS|MINGW' (uname); and command -s cygpath >/dev/null

Isn't the more stringent check needed? Under CYGWIN one might expect cygpath to be available, but is it guaranteed to be present under MSYS/MINGW?

@vsajip, it's not necessary. MSYS2 ships the Cygwin tool cygpath by default. MinGW as an environment requires MSYS2 to be installed. The codebase also uses cygpath unguarded at several points thus indicating that it is a requirement.

It's fine to add a guard clause as the uv devs requested, I don't see how it could hurt, but its also not required.