Use the `-s` option to shellcheck to ensure shell is correct.

[tee3]

It might be useful in some cases to use the -s option in shellcheck, maybe in all cases. This would use side information from Emacs (auto-mode-alist, etc.) to provide the right type of shell to the shellcheck.

I could also see issues with this approach since many shells are not supported by shellcheck, but I suppose that could be handled as well.