download: Switch savannah mirrors to HTTPS URLs.

download: Switch savannah mirrors to HTTPS URLs.

The URI scheme used for nongnu.freemirror.org needs to be consistent
between (guix download) and (guix gnu-maintenance) to make the simplified
savannah-updater (of a later commit) work.  While we're at it, switch the
other mirrors to https as well.

http://download.savannah.gnu.org/releases-noredirect/ is left unmodified
because it 404s

* download.scm (%mirrors)[savannah]: Switch from http to https where possible.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>