postgres: the provisioner dials the port the mesh gave the store

MESH_PROVISION_POSTGRES named 127.0.0.1:5432 literally; the seat twin
(${seat:mesh-store:5432}) corrected it only on the machine holding the
mesh-store seat. On any other machine — ace, where the module provides
postgres-database without the seat — the twin is empty and the sidecar
would have dialled whatever else holds 5432 (HAL's postgres). ${port:5432}
is the machine port on every node; on novox the twin still says the same
6852, so nothing moves there.

modules/postgres/module.json

@@ -105,7 +105,7 @@
         "/var/lib/postgres/superuser.secret:/run/secrets/superuser:ro"
       ],
       "env": {
-        "MESH_PROVISION_POSTGRES": "postgres://postgres@127.0.0.1:5432/postgres?sslmode=disable",
+        "MESH_PROVISION_POSTGRES": "postgres://postgres@127.0.0.1:${port:5432}/postgres?sslmode=disable",
         "MESH_PROVISION_POSTGRES_PORT": "${seat:mesh-store:5432}",
         "MESH_PROVISION_PASSWORD_FILE": "/run/secrets/superuser",
         "MESH_BROKER_FILE": "/run/secrets/broker",