Run mesh-knowledge's watch as the account the health tool reads as (hq issue 479)

The watch ran as root and wrote its verdict root's and 0600 into the state
directory, which the mesh gives to the account the runtime runs the bundle as;
knowledge_health could not open it, so the module read unhealthy with nothing
wrong. Run watch as that account, as zfs's watch does.

modules/mesh-knowledge/module.json

@@ -145,6 +145,7 @@
         "./mesh-knowledge",
         "watch"
       ],
+      "user": "${machine:account}",
       "env": {
         "DATABASE_URL_FILE": "${dir:state}/database.url",
         "MESH_KNOWLEDGE_STATUS": "${dir:state}/health.json"