Keeping the secrets dir read-only is more simple and robust.
- lnd seed mnemonic creation and joinmarket wallet creation can be
run as the regular service user instead of root.
- It is easier to switch to a third-party secrets deployment
method in the future.
Don't create a seed mnemonic for lnd when a wallet exists.
This avoids creating unused mnemonics and helps simplifying
the migration command in `versioning.nix`.
