prompt: Set LC_ALL instead of LANG

LC_ALL and LC_* override LANG, so if the user has any of them set,
__vte_urlencode would give the wrong output.

https://bugzilla.gnome.org/show_bug.cgi?id=709354
(cherry picked from commit 9c9f1451f37864f18e711c348f6335a22e7c4c4d)