fix(forge): force optimism and arbitrum to --slow mode

[joshieDo]

Motivation

ref #2002 (comment)

Force --slow on Arbitrum and Optimism. Both of don't have mempools like L1's so if you send multiple txs they get rejected with nonce too low.

Move RPC gas estimation right before sending the intended tx, and add a note to the output indicating the "total gas/ETH cost estimates" shown beforehand aren't guaranteed to be accurate (since we don't estimate Optimism costs correctly)

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Also removed the first script gas estimation if we have a RPC present, since it doesn't give any valuable information. Some users were also confused by the [expected] discrepancies on both.

[mattsse]

thanks for the docs!