jenkins: also test on bullseye (#43883)

Jenkinsfile

@@ -7,6 +7,7 @@ pipeline {
         stage('Unit Tests') {
             steps {
                 sh 'sudo ./test-nspawn buster'
+                sh 'sudo ./test-nspawn bullseye'
             }
         }
     }

test-nspawn

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -ex
 
-if [ $1 = 'buster' ]
+if [ $1 = 'buster' || $1 = 'bullseye' ]
 then
     suite=$1
     image=publik-devinst-$suite