only perform contract code check if non-hardcoded address is used

[noot]

as title says, we should only perform the contract code check on startup if the contract used isn't the one hard-coded into our network config. firstly I'm sick of redeploying the sepolia contract each time a comment is changed :p and also release-wise, we can't deploy the mainnet contract each time the contract has a minor update, so master would break if we code-check the hardcoded contract.