nixos node svc: use topology.json vs yaml for consistency w/ other cfgs

Author: johnalotoski

nix/nixos/cardano-node-service.nix

@@ -56,7 +56,7 @@ let
   selectTopology = i:
     if cfg.topology != null
     then cfg.topology
-    else toFile "topology.yaml" (toJSON (if (cfg.useNewTopology) then assertNewTopology i else oldTopology i));
+    else toFile "topology.json" (toJSON (if (cfg.useNewTopology) then assertNewTopology i else oldTopology i));
 
   topology = i:
     if cfg.useSystemdReload