github: Switch to downloading from ftpmirror.gnu.org

This should make the downloads more efficient; currently the
main ftp.gnu.org is very slow, feeding data at around 20 KB/s,
making the download of the GCC sources take up to an hour,
and using a mirror is more considerate in general.

Signed-off-by: Martin Storsjö <martin@martin.st>
1 file changed