nixos/syncoid: add possibility to use string type for sshKey options

The sshKey options do not need to be a valid path at build time. Using
string instead allow use case when the path is not known at build time
such as when using systemd credentials (e.g. `sshKey =
"\${CREDENTIALS_DIRECTORY}/zfs-replication_ed25519";`).

nixos/modules/services/backup/syncoid.nix

···
     };
 
     sshKey = mkOption {
-      type = types.nullOr types.path;
-      # Prevent key from being copied to store
-      apply = mapNullable toString;
+      type = with types; nullOr (coercedTo path toString str);
       default = null;
       description = lib.mdDoc ''
         SSH private key file to use to login to the remote system. Can be
···
           recursive = mkEnableOption (lib.mdDoc ''the transfer of child datasets'');
 
           sshKey = mkOption {
-            type = types.nullOr types.path;
-            # Prevent key from being copied to store
-            apply = mapNullable toString;
+            type = with types; nullOr (coercedTo path toString str);
             description = lib.mdDoc ''
               SSH private key file to use to login to the remote system.
               Defaults to {option}`services.syncoid.sshKey` option.