Fallback to the default Balancer Vault address during the deployment (#243)

## 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.

Author: squadgazzz

src/deploy/002_settlement.ts

@@ -7,6 +7,8 @@ import Vault from "../../balancer/Vault.json";
 import BALANCER_NETWORKS from "../../balancer/networks.json";
 import { CONTRACT_NAMES, SALT } from "../ts/deploy";
 
+const DEFAULT_VAULT_ADDRESS = "0xBA12222222228d8Ba445958a75a0704d566BF2C8";
+
 const deploySettlement: DeployFunction = async function ({
   deployments,
   ethers,
@@ -46,10 +48,13 @@ const deploySettlement: DeployFunction = async function ({
     >;
     const vaultDeployment = vaultNetworks[chainId];
     if (vaultDeployment === undefined) {
-      throw new Error(`Vault not deployed on chain ${chainId}`);
+      console.warn(
+        `Vault not deployed on chain ${chainId}, using default address.`,
+      );
+      vaultAddress = DEFAULT_VAULT_ADDRESS;
+    } else {
+      vaultAddress = vaultDeployment.address;
     }
-
-    vaultAddress = vaultDeployment.address;
   }
 
   await deploy(settlement, {