The three modules name their base rather than pinning a copy of it

Each named a digest produced inside a lab that no longer exists, so none of them
could be built anywhere else. They say which module they stand on now, and there
is deliberately no default — a build nobody told stops at the declaration rather
than at a reference that resolves to nothing.

modules/postgres/module.json

@@ -122,6 +122,13 @@
     }
   ],
   "build": {
+    "on": [
+      {
+        "arg": "RUNTIME_BASE",
+        "module": "mesh-tools",
+        "artifact": "runtime"
+      }
+    ],
     "artifacts": [
       {
         "name": "runtime",