ci: Use smaller CI images

Signed-off-by: Simon McVittie <smcv@collabora.com>
6 jobs for master
Status Job ID Name Coverage
  Test
passed #13824
build:bionic

00:11:11

passed #13823
build:buster

00:10:36

passed #13822
build:stretch

00:10:57

failed #13818
build:bionic

failed #13817
build:buster

failed #13816
build:stretch