Download GNU packages from mirrors

[adamjstewart]

Fixes #8983

Although https://ftp.gnu.org/ is back online now, we should still be downloading all GNU software from the officially recommended mirror URL. According to 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.

Hopefully http://ftpmirror.gnu.org/ is more stable than https://ftp.gnu.org/. This PR also changes many URLs from http to https, further improving security. I purged my downloads cache and confirmed that I can download all of these packages with the new URLs.

@healther @bassenj @tz-rrze