Maintenance: Fix packager.io tests for rhel-9

.gitlab/packager.io/scenarios/internal-services/rhel.sh

@@ -2,6 +2,9 @@
 
 set -eu
 
+# Workaround: postgresql fails on rhel-9 when it gets installed together with Zammad.
+dnf install -y postgresql-server
+
 dnf install -y zammad
 
 curl --retry 30 --retry-delay 1 --retry-connrefused http://localhost:3000 | grep "Zammad Helpdesk"