parallel-workload: quote the hedged-get delay flag's duration values

FlipFlags interpolates flag values into ALTER SYSTEM SET unquoted, so
duration-valued flags carry pre-quoted values in flags_with_values (as
persist_state_update_lease_timeout already does). The unquoted 10ms
value made every parallel-workload Nightly variant fail with a parse
error as soon as the flipper picked the delay flag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ceufn9nP1RbEJCtww1Ww8i

Author: ggevay

misc/python/materialize/parallel_workload/action.py

@@ -2799,7 +2799,11 @@ def __init__(
             "16",
         ]
         self.flags_with_values["persist_blob_hedged_get_enabled"] = BOOLEAN_FLAG_VALUES
-        self.flags_with_values["persist_blob_hedged_get_delay"] = ["0s", "10ms", "2s"]
+        self.flags_with_values["persist_blob_hedged_get_delay"] = [
+            "'0s'",
+            "'10ms'",
+            "'2s'",
+        ]
         self.flags_with_values["enable_variadic_left_join_lowering"] = (
             BOOLEAN_FLAG_VALUES
         )