Fix `test_genesis_hash_available` to be fill-only

Test `tests/frontier/opcodes/test_blockhash.py::test_genesis_hash_available` will fail when run with the `execute` command because it depends on `Op.BLOCKHASH(0)`, but since we only store the last `8191` hashes (or 256 prior to Prague), it will yield a different result.

We can add a skip marker to it during execute-mode:
```python
@pytest.mark.execute(pytest.mark.fill("Incompatible"))
def test_genesis_hash_available(blockchain_test: BlockchainTestFiller, pre: Alloc) -> None:
```

This can be deferred post-weld.