emacs: Enable 'guix-build-log-minor-mode' in shell buffers.

* emacs/guix-build-log.el (guix-build-log-minor-mode-activate): New variable.
  (guix-build-log-minor-mode-activate-maybe): New function.
* emacs/guix-init.el: Add it to 'shell-mode-hook'.
* doc/emacs.texi (Emacs Build Log): Mention it.