chore: change variable names to silence compiler warning - #88

Compiler run successful with warnings:
Warning (2519): This declaration shadows an existing declaration.
   --> src/GuardedExecutor.sol:487:46:
    |
487 |         returns (SpendInfo[][] memory spend, bytes32[][] memory canExecute)
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Note: The shadowed declaration is here:
   --> src/GuardedExecutor.sol:395:5:
    |
395 |     function canExecute(bytes32 keyHash, address target, bytes calldata data)
    |     ^ (Relevant source part starts here and spans across multiple lines).