ci(benchmarks): use json.dumps to pass data from configs to test runs (#13844)

this fixes an issue with the startup benchmarks not being able to
properly set env variables

## Checklist
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- The change includes or references documentation updates if necessary
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## Reviewer Checklist
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changes
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Author: brettlangdon

benchmarks/base/run.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
 
+import json
 import os
 import subprocess
 import sys
@@ -45,7 +46,11 @@ def run(scenario_py, cname, cvars, output_dir):
     ]
     for cvarname, cvarval in cvars.items():
         cmd.append("--{}".format(cvarname))
-        cmd.append(str(cvarval))
+        if isinstance(cvarval, (dict, list)):
+            # convert dicts and lists to JSON strings
+            cmd.append(json.dumps(cvarval))
+        else:
+            cmd.append(str(cvarval))
 
     proc = subprocess.Popen(cmd)
     proc.wait()