matrix: Reduce postgres_maintenance_work_mem

Still seeing PostgreSQL session processes eating over 600M of private
memory. What's going on?

playbooks/matrix.archlinux.org.yml

@@ -16,7 +16,7 @@
     - role: postgres
       postgres_shared_buffers: 1GB
       postgres_work_mem: 32MB
-      postgres_maintenance_work_mem: 256MB
+      postgres_maintenance_work_mem: 32MB
       postgres_effective_cache_size: 2GB
     - role: postfix
       postfix_relayhost: "mail.archlinux.org"