chore: fix linter issue

Author: pyrooka

ibm_cloud_sdk_core/token_managers/container_token_manager.py

@@ -115,7 +115,8 @@ def retrieve_cr_token(self) -> str:
             return cr_token
         # pylint: disable=broad-except
         except Exception as ex:
-            raise Exception('Unable to retrieve the CR token value from file {}: {}'.format(cr_token_filename, ex))
+            raise Exception(
+                'Unable to retrieve the CR token value from file {}: {}'.format(cr_token_filename, ex)) from None
 
     def request_token(self) -> dict:
         """Retrieves a CR token value from the current compute resource,