Fix deprecation warning with elixir 1.17

Elixir 1.17 deprecated single quotes for charlists and a single-quote charlist is currently used in a System.cmd call. However there’s no reason to use a charlist here and in fact the function specs demands binaries be used here ever since at least Elixir 1.0.4, possibly always, so let’s follow that.

See 1.0.4’s function spec which apart from adding parentheses still matches the one from 1.17.1

cmd(binary, [binary], Keyword.t) :: {Collectable.t, exit_status :: non_neg_integer}