bench: WalletMigration benchmark broken

master @ cdc32994feadf3f15df3cfac5baae36b4b011462
```bash
export CC=clang
export CXX=clang++
cmake -B build -DBUILD_BENCH=ON
cmake --build build
./build/bin/bench_bitcoin -filter=WalletMigration
bench_bitcoin: wallet/wallet.cpp:1772: void wallet::CWallet::InitWalletFlags(uint64_t): Assertion `m_wallet_flags == 0' failed.
Aborted (core dumped)
```