ci: Use smaller CI images

Signed-off-by: Simon McVittie <smcv@collabora.com>
3 jobs for wip/smcv/ci-slim in 12 minutes and 28 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed build:bionic #13815

00:12:26

passed build:buster #13814

00:12:01

passed build:stretch #13813

00:12:19