`CWallet::GetEncryptionKey()` would return a reference to the internal
`CWallet::vMasterKey`, guarded by `CWallet::cs_wallet`, which is unsafe.

Returning a copy would be a shorter solution, but could have security
implications of the master key remaining somewhere in the memory even
after `CWallet::Lock()` (the current calls to
`CWallet::GetEncryptionKey()` are safe, but that is not future proof).

So, instead of `EncryptSecret(m_storage.GetEncryptionKey(), ...)`
change the `GetEncryptionKey()` method to provide the encryption
key to a given callback:
`m_storage.WithEncryptionKey([](const CKeyingMaterial& k) { EncryptSecret(k, ...); })`

This silences the following (clang 18):

wallet/wallet.cpp:3520:12: error: returning variable 'vMasterKey' by reference requires holding mutex 'cs_wallet' [-Werror,-Wthread-safety-reference-return]
 3520 |     return vMasterKey;
      |            ^

ninja all check-all