gitlab-ci: Use "make defconfig" for image build too

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
2 jobs for gitlab-ci-deb in 43 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #9578
librem5
build:aarch64:Image

00:43:14

passed #9579
librem5
build:aarch64:deb

00:32:37