Use http instead of https for the gnu ftp mirror

https seems broken on the Debian image:

START   autotools/hello/42930621-fetch.499.log
START   Fetching https://ftpmirror.gnu.org/gnu/automake/automake-1.16.tar.gz
FAILURE Fetching https://ftpmirror.gnu.org/gnu/automake/automake-1.16.tar.gz
FAILURE tar source at hello.bst [line 16 column 2]: Error mirroring https://ftpmirror.gnu.org/gnu/automake/automake-1.16.tar.gz: <urlopen error [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:720)>