Maintain original read concern for $currentOp in passthrough suites that change read concern

With SERVER-47603 the db.currentOp() shell helper uses the aggregation $currentOp stage instead of calling the currentOp server command. Both only support read concern "local", however passthrough suites that change read concern only change the read concern of the aggregation command and maintain the original read concern of the currentOp command. With the change from the server command to $currentOp, some jstests that use the db.currentOp() shell helper had the assumes_read_concern_unchanged blocklist tag added. To add this test coverage back, we should skip changing an aggregate command's read concern in passthroughs that change read concern if it has a $currentOp stage.