fix: add missing json import to chonk-breakdowns dashboard

PR #18987 moved the CI dashboard from iac repo to aztec-packages but
the json import was missing, causing 500 errors on the flows API.

Author: johnathan79717

ci3/dashboard/rk.py

@@ -2,6 +2,7 @@
 from flask_compress import Compress
 from flask_httpauth import HTTPBasicAuth
 import gzip
+import json
 import os
 import re
 import requests