Prevent shfmt from running when using the fish variant of shell-script-mode

shfmt entirely reasonably does not plan to support fish (https://github.com/mvdan/sh/issues/405) which means that when apheleia runs `shfmt` in `shell-script-mode`, the reformatting is not accurate. It would be ideal if apheleia could limit `shfmt` to scripts written in a shell language that `shfmt` supports.