Fish activator passes VIRTUAL_ENV through cygpath

[LuNoX]

Closes #3160. Sourcing activate.fish from fish run inside MSYS2 (or similar) breaks the PATH because of a path style mismatch. This PR applies the changes from #1970 as well as the change to using uname from #3036 to the fish template as well.

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)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[LuNoX]

Concerning the PR checklist:

• As far as I understand the linter is for the python style correct? I only touched the fish script so should I still run the linter?
• For test coverage, I would appreciate some guidance as I'm not sure how to expand the fish activation unit tests. They currently do skip the test if the system IS_WIN. Do you want a completely new test, or just remove the skip, or add new test thats covering for MSYS2, Cygwin and MinGW fish shells?
• As for docs, I don't believe anything needs to be updated, as the change brings the repo in line with what the docs promise (activating inside fish regardless of Unix or windows just works)

[gaborbernat]

LGTM