CI: fix install on bullseye

We should use bullseye-backports.

Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
6 jobs for master in 5 minutes and 25 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Test
passed dispatcher/install/bullseye #154734
amd64

00:04:05

passed dispatcher/upgrade/bullseye #154736
amd64

00:04:13

passed server/install/bullseye #154730
amd64

00:04:27

passed server/upgrade/bullseye #154732
amd64

00:04:51

 
  Publish
skipped failure #154737
amd64
passed success #154738
amd64

00:00:31