emacs: Add "Build Log" button to Package Info.

* emacs/guix-main.scm (package-build-log-file): New procedure.
* emacs/guix-ui-package.el (guix-package-build-log-file)
(guix-package-find-build-log)
(guix-package-info-insert-build-log-button): New procedures.
(guix-package-info-button-functions): Add
'guix-package-info-insert-build-log-button'.