Just set default-process-coding-system to '(utf-8 . utf-8). You can do this by adding this to your init file:
(setq default-process-coding-system '(utf-8 . utf-8))
This and other useful variables are described in chapter 33.10.5 Default Coding Systems of the Emacs Lisp manual. You may also be interested in the process-coding-system-alist variable.
You can ...