Auto merge of #26998 - gterzian:improve_task_cancelling, r=jdm

Use a stronger atomic guarantee when cancelling tasks

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In the context of https://github.com/servo/servo/issues/22507

Note that the "other side" of these operations is at https://github.com/servo/servo/blob/0b61cfc3ae803ac0f9deef937f890f83b24c9a35/components/script/task.rs#L102

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