overseer: spammer basefee is hard-coded

[merklefruit]

based-op/based/crates/common/src/signing.rs

Line 122 in a17cebc

let max_fee_per_gas = 10_000_000;

As the fee is hard-coded for transactions sent by the overseer spammer, it doesn't work if the basefee has been increased by other usage of the chain (external spammers, or overseer itself).

Using the previous block's basefee is probably more ideal. The basefee can be fetched once per L2 block (2s) as to avoid wasting time fetching it every time we send a transaction.