`g:fzf_action` doesn't accept functions

- [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

`a.vim`:

```vim
function! s:build_quickfix_list(lines)
call setqflist(map(copy(a:lines), '{ "filename": v:val }'))
copen
cc
endfunction

let g:fzf_action = {
\ 'ctrl-q': function('s:build_quickfix_list')}
```

Not Alpine Linux-specific.

```
$ docker run --rm -itv $PWD/a.vim:/a.vim alpine
/ # apk add vim bash curl git ripgrep bat
/ # bash
bash-5.1# TERM=xterm-256color
bash-5.1# cd /usr/share/vim
bash-5.1# vim -Nu <(curl https://gist.githubusercontent.com/junegunn/6936bf79fedd3a079aeb1dd2f3c81ef5/raw)
:so /a.vim
:Rg
ctrl-q
:copen
```


The `build_quickfix_list` solution was suggested [here][a] and [here][c]. It doesn't work because for some reason non-strings are [not accepted][b].

**UPD** I've found the [change][d]. I don't get the description, but in `fzf.vim` actions only ever received the [first filename][e]. And it's kind of confusing:

https://github.com/junegunn/fzf.vim/issues/621
https://github.com/junegunn/fzf.vim/pull/867
https://github.com/junegunn/fzf.vim/issues/185#issuecomment-322120216
https://github.com/junegunn/fzf/blob/a74731d/README-VIM.md#examples

[a]: https://github.com/junegunn/fzf/blob/a74731d/README-VIM.md#examples
[b]: https://github.com/junegunn/fzf.vim/blob/a4ce66d/autoload/fzf/vim.vim#L300
[c]: https://github.com/junegunn/fzf.vim/issues/185#issuecomment-322120216
[d]: https://github.com/junegunn/fzf.vim/commit/b9b275a897e3cc5f900bf802ca74e3442f9027d4
[e]: https://github.com/junegunn/fzf.vim/commit/f13853cc8dbf019652051eda1f3be2a537dac794#diff-f697bec8f1ba94d14196b753546e9fe6a486a33e10f25afb42765573c991fa0fR221

Start with autoload/fzf/vim.vim around the linked line where actions are handled, then reproduce the behavior using the a.vim configuration and the Rg command described in the issue. Compare the referenced fzf.vim and fzf changes to understand how g:fzf_action values are processed and verify that the function-based quickfix action works as intended.

Written by the indexing model from the issue text.