Download GNU deps from mirror

[Frans de Jonge]

This is a simple one. https://ftp.gnu.org/ is currently offline but we shouldn't actually be using it anyway.

https://www.gnu.org/server/mirror.en.html

First, for users/downloaders: the address http://ftpmirror.gnu.org/ multiplexes between the mirrors, trying to choose one that is nearby and up to date. E.g., http://ftpmirror.gnu.org/emacs/ goes to a mirror's directory of GNU Emacs. We recommend using this generic ftpmirror.gnu.org address wherever possible in links, documentation, etc., to reduce load on the main GNU server.

tl;dr http://ftpmirror.gnu.org should be more stable.