Option --copy-host-keys should allow a custom destination

### Prerequisites - [x] I have searched existing issues to make sure this isn't a duplicate - [x] I have checked the documentation to see if this feature already exists ### Problem Description I am using both sops-nix and impermanence, which wipes the root partition on boot, only saving the files in `/persist`. When enabled, writing the host's ssh keys to `/mnt/etc/ssh/ssh_host_*` isn't enough, as they are wiped immediately, the directory is bind mounted to `/persist/etc/ssh`, and then new keys are written. ### Proposed Solution Allow `--copy-host-keys` to have a `<destination>` option, defaulting to `/etc/ssh`, allowing for an override to a persistent directory, such as `/persist/etc/ssh`. ### Alternatives Considered _No response_ ### Feature Type New command-line option ### Use Case Impermanence / NixOS persistence setups on new machines ### Implementation Ideas _No response_ ### Additional Context _No response_