chore: improve multisig builder transaction simulation building

[jackchuma]

Previously, our transaction simulation for MultisigBuilder setups required a workaround. To simulate a series of transactions through a hierarchy of multisigs using a single multicall, we had to temporarily change the ownership of the very first multisig in the chain to be solely the multicall contract. This allowed the multicall contract to initiate the first approval, which then cascaded up the chain, with each child multisig approving its parent, until the final multisig executed the actual task call(s). While this worked for most simulations, it prevented us from directly testing changes to the real signers of a multisig because our workaround had removed them. This update removes that ownership change. Instead, it directly grants an approval in the initial multisig on behalf of msg.sender, achieving the same simulation capability without losing the ability to manipulate actual signers.

[cb-heimdall]

✅ Heimdall Review Status

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[mdehoog]

A potentially nicer syntax would be DoubleNestedMultisigBuilder(address(this)).sign(safe1, safe3);