This PR addresses a possible warning when an error occurs while processing the wp action-scheduler action cancel --all command.

Closes #1268.

Testing Instructions

1. Temporarily enable the following snippet on your site to simulate an error occurring while bulk-canceling actions:

   <?php
   add_action(
       'action_scheduler_bulk_cancel_actions',
       function() {
           throw new \Exception( 'Some error' );
       }
   );

2. Run the following commands:

   $ wp action-scheduler action create myhook async
   $ wp action-scheduler action cancel myhook --all
   

3. Confirm no new PHP warning is logged.
4. (Optional) Repeat the above on trunk and confirm that the following error appears in your logs:

   PHP Warning: Undefined variable $multiple in /[...]/action-scheduler/classes/WP_CLI/Action/Cancel_Command.php on line 82