emacs: Disable minor modes in 'guix-build-log-mode'.

* emacs/guix-build-log.el (guix-build-log-mode-hook): Remove
  'compilation-shell-minor-mode' as it is slow on big buffers, and
  'view-mode' as it rebinds too many keys.
  (guix-build-log-mode-map): Add "c" and "v" key bindings for the
  removed minor modes.