warp analyze contains invalid header information

when running `warp analyze` I noted that jq queries are failing due to some non json characters printed in the beginning of the output:

```
~/benchmarks/tls-kms$ warp analyze objsize-100MiB-threads-80-delete-25-02-21-01-20.json.zst --json| head
Loading "objsize-100MiB-threads-80-delete-25-02-21-01-20.json.zst"{
  "v": 2,
```

(the 'loading <foo>.zst part)

