Fix CI: create PG 18 cluster before starting

pg_createcluster needed since PGDG apt doesn't auto-create on CI runners.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jhf

.github/workflows/regression.yml

@@ -18,7 +18,7 @@ jobs:
 
       - name: Start PostgreSQL 18
         run: |
-          sudo pg_ctlcluster 18 main start
+          sudo pg_createcluster 18 main --start
           sudo -u postgres createuser -s $(whoami)
 
       - name: Add pg_config to PATH