[vim] Don't use :terminal on msys2 or Cygwin (junegunn#1155)

Close junegunn#1152

msys2 terminal Vim assumes that it runs in mintty
so `:terminal` uses `TERM=xterm`.
fzf doesn't support `TERM=xterm` on Windows.

plugin/fzf.vim

@@ -392,7 +392,7 @@ try
   let has_vim8_term = has('terminal') && has('patch-8.0.995')
   let has_nvim_term = has('nvim-0.2.1') || has('nvim') && !s:is_win
   let use_term = has_nvim_term ||
-    \ has_vim8_term && (has('gui_running') || s:is_win || !use_height && s:present(dict, 'down', 'up', 'left', 'right', 'window'))
+    \ has_vim8_term && !has('win32unix') && (has('gui_running') || s:is_win || !use_height && s:present(dict, 'down', 'up', 'left', 'right', 'window'))
   let use_tmux = (!use_height && !use_term || prefer_tmux) && !has('win32unix') && s:tmux_enabled() && s:splittable(dict)
   if prefer_tmux && use_tmux
     let use_height = 0