Use file sink encoding `json`

Encoding ndjson isn't supported in recent versions.

nixos/tests/vector.nix

···
               type = "file";
               inputs = [ "journald" ];
               path = "/var/lib/vector/logs.log";
-              encoding = { codec = "ndjson"; };
+              encoding = { codec = "json"; };
             };
           };
         };