[nix] Modularized the container backend so I can easily switch it with an option

system/hosts/mcp/containers/havenisms.com/chat.nix

@@ -8,10 +8,10 @@ in
 
   sops.secrets = {
     "matrix/syncv3/db-password" = {
-      restartUnits = [ "podman-matrix-sliding-sync.service" ];
+      restartUnits = [ "${config.local.container-backend}-matrix-sliding-sync.service" ];
     };
     "matrix/syncv3/secret" = {
-      restartUnits = [ "podman-matrix-sliding-sync.service" ];
+      restartUnits = [ "${config.local.container-backend}-matrix-sliding-sync.service" ];
     };
   };