ci: allow passing a dict/list as a step parameter

In some cases we may want to pass a complex value to a step. For
example:

  .buildkite/pipeline_pr.py \
    --step-param 'retry/automatic=[{"exit_status": "*", "limit": 2}]'

(cherry picked from commit c6add4f)
Signed-off-by: Pablo Barbáchano <[email protected]>

Author: pb8o

.buildkite/common.py

@@ -6,6 +6,7 @@
 """
 
 import argparse
+import ast
 import json
 import os
 import random
@@ -133,7 +134,7 @@ def __call__(self, parser, namespace, value, option_string=None):
         res = getattr(namespace, self.dest, {})
         key_str, val = value.split("=", maxsplit=1)
         keys = key_str.split("/")
-        update = {keys[-1]: val}
+        update = {keys[-1]: ast.literal_eval(val)}
         for key in list(reversed(keys))[1:]:
             update = {key: update}
         res = overlay_dict(res, update)