nixos/kanidm: add home_mount_prefix to BindPaths if set (#382915)

          User = "kanidm";

          Group = "kanidm";

          BindPaths = [

          BindPaths =

            [

              # To create the socket

              "/run/kanidmd:/run/kanidmd"

              # To store backups

              cfg.serverSettings.online_backup.path

          ];

            ]

            ++ optional (

              cfg.enablePam && cfg.unixSettings ? home_mount_prefix

            ) cfg.unixSettings.home_mount_prefix;

          AmbientCapabilities = [ "CAP_NET_BIND_SERVICE" ];

          CapabilityBoundingSet = [ "CAP_NET_BIND_SERVICE" ];