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

[offbyone]

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.