emacs: Add "View build log" action to build popup.

* emacs/guix-command.el (guix-run-view-build-log): New function.
  (guix-command-additional-execute-arguments,
  guix-command-special-executors): Add entries for "View build log"
  action.
* emacs/guix-utils.el (guix-find-file-or-url): New function.