gitlab-ci: use stable-backports for stable

3 jobs for master in 3 minutes and 30 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
failed #132962
stable

00:03:29

passed #132961
testing

00:02:35

passed #132960
unstable

00:02:33

 
Name Stage Failure
failed
stable Test
  libsigsegv2 libstdc++-6-dev libtimedate-perl libtool libtsan0 libubsan0
libunistring0 libxml2 linux-libc-dev m4 make man-db patch po-debconf python3
python3-minimal python3.5 python3.5-minimal shunit2 xz-utils
The following packages will be DOWNGRADED:
libc6
0 upgraded, 77 newly installed, 1 downgraded, 0 to remove and 6 not upgraded.
E: Packages were downgraded and -y was used without --allow-downgrades.
E: Failed to process build dependencies
ERROR: Job failed: exit code 1