fzf preview not working when set automount in WSL on Windows

- [x] I have fzf 0.23.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

Setup
- neovim v0.5.0-dev+1175-g1186f7dd9 through `nvim-qt.exe` (GUI version of neovim)
- Windows 10 build 19042.867
- WSL running Ubuntu 20.04 LTS

When I have an automount set for WSL (to simplified the path from having `/mnt` prefix),

```
File: /etc/wsl.conf
----------------------------------
[automount]
root = /
options = "metadata"
```

fzf preview always complain about cannot find the correct preview script.

```
/bin/bash: /mnt/c/Users//AppData/Local/nvim/plugged/fzf.vim/bin/preview.sh: No such file or directory
```

However, when I apply this patch...

```diff
--- a/autoload/fzf/vim.vim
+++ b/autoload/fzf/vim.vim
@@ -170,7 +170,7 @@ function ! fzf#vim#with_preview(...)
let $WSLENV = 'MSWINHOME/u:'.$WSLENV
endif
let preview_cmd = 'bash '.(is_wsl_bash
- \ ? substitute(substitute(s:bin.preview, '^\([A-Z]\):', '/mnt/\L\1', ''), '\', '/', 'g')
+ \ ? substitute(substitute(s:bin.preview, '^\([A-Z]\):', '/\L\1', ''), '\', '/', 'g')
\ : escape(s:bin.preview, '\'))
else
let preview_cmd = fzf#shellescape(s:bin_preview)
```

I got it working with nothing changed.

I think rather than having fzf.vim figure out if the automount is set automatically, it would be better with a flag set in the configuration instead. This should simplified the change needed in order to have this working. WDYT?