CI: fix install on bullseye

We should use bullseye-backports.

Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
6 jobs for master in 4 minutes and 13 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Test
passed dispatcher/install/bullseye #153935
amd64

00:03:01

passed dispatcher/upgrade/bullseye #153937
amd64

00:03:10

passed server/install/bullseye #153931
amd64

00:03:25

passed server/upgrade/bullseye #153933
amd64

00:03:44

 
  Publish
skipped failure #153938
amd64
passed success #153939
amd64

00:00:26