Querier is passed to Migrate by reference

A reference to the [querier](https://github.com/CosmWasm/wasmd/blob/main/x/wasm/keeper/keeper.go#L571) is passed to [Migrate](https://github.com/CosmWasm/wasmd/blob/main/x/wasm/keeper/keeper.go#L576) (`&querier`) as opposed to other wasmVM methods like [Execute](https://github.com/CosmWasm/wasmd/blob/main/x/wasm/keeper/keeper.go#L420) that receive `querier` as-is. Both `Execute` and `Migrate` expect the same querier type in their [interface](https://github.com/CosmWasm/wasmd/blob/main/x/wasm/types/wasmer_engine.go#L104) declaration.

Is there a reason for this discrepancy?