Respect the user's shell variable

- Category
- [x] Question
- [ ] Bug
- [ ] Suggestion
- OS
- [x] Linux
- [ ] Mac OS X
- [ ] Windows
- [ ] Etc.
- Vim
- [ ] Vim
- [x] Neovim

I'm using the fish shell but I'm setting

```
set shell=/bin/bash
```

in my Neovim config. Using FZF it seems like the commands are getting executed under Fish. I noticed this since I started to have problems with FZF's autocompletion (sometimes cursors inserted only control command but nothing happened, sometimes even ctrl-k/j didn't work - can't reproduce outside of Neovim) which went away when starting Neovim with `SHELL=/bin/bash nvim`. I _think_ this might be due to how FZF picks up the user's shell here:

https://github.com/junegunn/fzf/blob/1acd2adce271ffaf0bfb303c6dc27fe2b03c9e04/src/util/util.go#L103

Should `fzf.vim` respect the user's shell variable?