Fallback to the default Balancer Vault address during the deployment

[squadgazzz]

Description

This PR updates the GPv2Settlement SC deployment script to default to the deterministic Balancer Vault address(0xBA12222222228d8Ba445958a75a0704d566BF2C8) when it is not actually deployed yet.

While the GPv2Settlement contract accepts a Balancer Vault address in its constructor, it does not strictly require the Vault to be deployed for basic ERC20 settlement functionality. This was confirmed in internal discussion:

• On Gnosis Chain, we used the Settlement contract before the Balancer Vault was even available.
• The contract only interacts with the Vault for advanced balance sources (non-ERC20 tokens), which are not required for standard usage.
• Vault-dependent logic is guarded and will not be triggered unless certain preconditions (e.g. balanceOpCount > 0) are met

Test Plan

Created an Avalanche fork on Tenderly before the Balancer Vault deployment, and the deployment worked well: https://dashboard.tenderly.co/cow-protocol/barn/testnet/14090eef-bc20-43fc-994d-2353743159cb

Potential risks

SAFE core contract addresses on ZK rollups differ from those on optimistic ones. Not sure if there is a chance that the Balancer Vault will be deployed later with a different address.

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[squadgazzz]

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