backport: fix(vim.fs): dirname() returns "." on mingw/msys2 (#30484)

fix(vim.fs): dirname() returns "." on mingw/msys2 #30480

Problem:
`vim.fs.dirname([[C:\User\XXX\AppData\Local]])` returns "." on
mingw/msys2.

Solution:
- Check for "mingw" when deciding `iswin`.
- Use `has("win32")` where possible, it works in "fast" contexts since
  b02eeb6a72.

runtime/lua/provider/python/health.lua

@@ -1,5 +1,5 @@
 local health = vim.health
-local iswin = vim.uv.os_uname().sysname == 'Windows_NT'
+local iswin = vim.fn.has('win32') == 1
 
 local M = {}