emacs: Fix 'guix-build-log-phase-toggle-all'.

* emacs/guix-build-log.el (guix-build-log-phase-toggle-all): Handle the
  case when there are no characters before the first phase:
  'guix-build-log-next-phase' moves to the second phase in such a case,
  and the toggling is not done properly, so use 're-search-forward'
  instead.