tools/pdf: only prompt to build epdfinfo once
tools/pdf: only prompt to build epdfinfo once

Fixes regression mentioned in #1788

hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs