Update shellcheck and shfmt to the latest version.
Fixes <https://github.com/koreader/koreader/issues/5152>.
Btw, you can apply shellcheck suggestions with a command like:
```
shellcheck --include=SC2250 -f diff *.sh | git apply
```
Although the current hard-coded paths work on debian, they may fail on many other distros. Other distros may pack koreader based on the released deb file. I personally have problem using current script on nixos, as it does not use the paths `/bin/bash` and `/usr/lib/`.
