cpp: Update key with --with-secret instead of updating it twice

* lang/cpp/src/key.cpp (Key::update): Call Context::key() only once
with KeyListMode::WithSecret.

With gpg >= 2.1, get the key with --with-secret instead of first trying
to get the secret key and, if that fails, getting the public key.

• GnuPG-bug-id: T4794