modules/backup: add restic cache

modules/backup/default.nix

@@ -41,6 +41,7 @@ let
         export RESTIC_PASSWORD_FILE=${jobPasswordFile}
         export RESTIC_REPOSITORY="rest:https://${targetUsername}:$(cat ${targetPasswordFile})@${targetOptions.serverName}${repoPath}"
         export RESTIC_PROGRESS_FPS=0.1
+        export RESTIC_CACHE_DIR=/var/cache/restic
 
         restic snapshots || restic init
 
@@ -125,5 +126,8 @@ in
   config = mkIf cfg.enable {
     systemd.services = backupServiceUnits;
     systemd.timers = backupServiceTimers;
+    systemd.tmpfiles.rules = [
+      "d /var/cache/restic - - - - -"
+    ];
   };
 }