fix storage access inconsistency in ERC20 transfer test

`erc20` transfer test fails with "Insufficient balance" despite storage showing correct balance value. balance is accessible via `balances()` call but not during transfer execution. see https://github.com/zink-lang/zink/blob/bdff1f55742025f59201e85a5809310da6945836/examples/erc20.rs#L257

this blocks full testing of the `erc20` impl.