Fix TypeError when using config_expr with @trigger events

Fixes #3080

Author: RONAK-AI647

metaflow/plugins/events_decorator.py

@@ -104,6 +104,7 @@ def process_event(self, event):
         if is_stringish(event):
             return {"name": str(event)}
         elif isinstance(event, dict):
+            event = dict(event)  # make a copy since we will modify the dict
             if "name" not in event:
                 raise MetaflowException(
                     "The *event* attribute for *@trigger* is missing the *name* key."