ci: verify JSON vector formatting

By running `tools/reformat_json.py` and verifying no diff is produced.

Author: cpu

.github/workflows/vectorlint.yml

@@ -29,10 +29,13 @@ jobs:
             exit 1
           fi
 
-      - name: Verify formatting
+      - name: Verify Go formatting
         # 'gofmt -l .' to list files whose formatting differs from gofmt's
         # 'test -z' to verify that the output is empty
         run: test -z "$(gofmt -l .)"
 
+      - name: Verify JSON formatting
+        run: ./tools/reformat_json.py && git diff --exit-code
+
       - name: Run vectorlint
         run: go run ./tools/vectorlint