Remove unnecessary tx hash complexity and make it version agnostic

.env.example

@@ -1,4 +1,5 @@
 SAFE=0x0000000000000000000000000000000000000000
 SENDER=0x0000000000000000000000000000000000000000
 
+SAFE_NONCE=
 FOUNDRY_ETH_RPC_URL=https://eth.llamarpc.com

README.md

@@ -5,6 +5,7 @@
 - Install [Foundry](https://github.com/foundry-rs/foundry).
 - Run `make` to initialize the repository.
 - Create a `.env` file from the template [`.env.example`](./.env.example) file.
+  - Use the environment variable `SAFE_NONCE` to override a transaction's nonce. Leave it blank to use the default, latest Safe nonce.
 
 You can customize the RPC url used in [`foundry.tml`](./foundry.toml) under the `rpc_endpoint` section. This is useful if your Safe is not deployed on mainnet (which is the default chain used).
 

script/SafeTxDataBuilder.sol

@@ -45,7 +45,7 @@ contract SafeTxDataBuilder is Script, SignatureDecoder {
     constructor(address payable safe) {
         SAFE = GnosisSafe(safe);
 
-        NONCE = SAFE.nonce();
+        NONCE = vm.envOr("SAFE_NONCE", SAFE.nonce());
         THRESHOLD = SAFE.getThreshold();
         DOMAIN_SEPARATOR = SAFE.domainSeparator();
     }
@@ -65,25 +65,18 @@ contract SafeTxDataBuilder is Script, SignatureDecoder {
     }
 
     function hashData(SafeTxData memory txData) internal view returns (bytes32) {
-        bytes32 safeTxHash = keccak256(
-            abi.encode(
-                SAFE_TX_TYPEHASH,
-                txData.to,
-                txData.value,
-                keccak256(txData.data),
-                txData.operation,
-                txData.safeTxGas,
-                txData.baseGas,
-                txData.gasPrice,
-                txData.gasToken,
-                txData.refundReceiver,
-                NONCE
-            )
+        return SAFE.getTransactionHash(
+            txData.to,
+            txData.value,
+            txData.data,
+            txData.operation,
+            txData.safeTxGas,
+            txData.baseGas,
+            txData.gasPrice,
+            txData.gasToken,
+            txData.refundReceiver,
+            NONCE
         );
-
-        bytes memory txHashData = abi.encodePacked(bytes1(0x19), bytes1(0x01), DOMAIN_SEPARATOR, safeTxHash);
-
-        return keccak256(txHashData);
     }
 
     function decode(bytes32 dataHash, bytes memory signature)