nixos/restic: change type of timerConfig option to attrsOf unitOption

This is needed for correctly passing the option to "systemd.timer"

nixos/modules/services/backup/restic.nix

@@ -1,6 +1,11 @@
 { config, lib, pkgs, ... }:
 
 with lib;
+
+let
+  # Type for a valid systemd unit option. Needed for correctly passing "timerConfig" to "systemd.timers"
+  unitOption = (import ../../system/boot/systemd-unit-options.nix { inherit config lib; }).unitOption;
+in
 {
   options.services.restic.backups = mkOption {
     description = ''
@@ -47,7 +52,7 @@ with lib;
         };
 
         timerConfig = mkOption {
-          type = types.attrsOf types.str;
+          type = types.attrsOf unitOption;
           default = {
             OnCalendar = "daily";
           };