gitlab-ci: Use prebuilt pipeline

This gives us lintin and autopkgtests without effort (but at
the cost of an additional build stage).
11 jobs for prebuilt-pipeline in 21 minutes and 14 seconds (queued for 42 seconds)
Status Name Job ID Coverage
  Build
passed api-visibility #239446
librem5

00:02:48

passed build-debian-clang #239449
librem5

00:04:17

passed build-debian-gcc #239447
librem5

00:03:10

passed build-debian-gcc-static #239448
librem5

00:03:10

passed build-fedora-gcc #239450
librem5

00:03:33

 
  Test+Docs
passed build-gtkdoc #239451
librem5

00:03:10

passed unit-test #239452
librem5

00:03:29

44.5%
 
  Package
passed package-debian-buster #239453
librem5

00:03:42

passed package-debian-buster:arm64 #239454
librem5:arm64

00:06:05

 
  Test Package
passed autopkgtest-debian-buster-package #239455
librem5

00:03:08

passed lintian-debian-buster-package #239456
librem5

00:01:06