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

nix/nixos/cardano-node-service.nix

@@ -56,11 +56,11 @@ 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
-    then "/etc/cardano-node/topology-${toString i}.yaml"
+    then "/etc/cardano-node/topology-${toString i}.json"
     else selectTopology i;
 
   mkScript = cfg:
@@ -616,7 +616,7 @@ in {
           if only the topology file has changed and p2p is in use.
 
           Cardano-node topology files will be stored in /etc as:
-            /etc/cardano-node/topology-''${toString i}.yaml
+            /etc/cardano-node/topology-''${toString i}.json
 
           Enabling this option will also allow direct topology edits for tests when a full
           service re-deployment is not desired.
@@ -781,7 +781,7 @@ in {
       };
 
       environment.etc = mkIf cfg.useSystemdReload (foldl'
-        (acc: i: recursiveUpdate acc {"cardano-node/topology-${toString i}.yaml".source = selectTopology i;}) {}
+        (acc: i: recursiveUpdate acc {"cardano-node/topology-${toString i}.json".source = selectTopology i;}) {}
       (range 0 (cfg.instances - 1)));
 
       ## TODO:  use http://hackage.haskell.org/package/systemd for: