I'm afraid you can't. Try and cast it to a sequence:
action.codeElement->asSequence()
but I don't think anything guarantees that the sequence you get will always be sorted in the same order.
If the metamodel is made that way, there should be a reason, so either you can contact the metamodel authors to check this reason, or you should sort the result of action.codeElement with some stable criterion:
action.codeElement->sortedBy( some OCL expression)