Bug report

Bug description:

When running source .venv/Scripts/activate.fish on windows from a fish shell inside msys2 ucrt64 it breaks the PATH.
Equivalently to gh-103325 activating a venv inside fish (from Cygwin, MSYS or MinGW) on windows will lead to mixing Unix and Windows path styles which then has knockon effects like breaking the PATH when

To reproduce:

• Boot a Windows partition
• Install Cygwin, MSYS2, MinGW or equivalent
• Install fish from inside Cygwin, MSYS2, MinGW or equivalent
• Install python
• cd into a python project of your choice
• Run your installed python's venv command
• Run echo $PATH
• Run source .venv/Scripts/activate.fish
• Run echo $PATH

Notice that the second PATH output contains a new broken/mismatched entry at the start which will in turn break all future uses of PATH.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-151042: venv: Pass VIRTUAL_ENV through cygpath inside fish on Windows #151043