fix permissions of /nix/persist/private, to be user-writable

this is important for my rsync-net backup scripts, which need to record timestamps in there

modules/persist/stores/private.nix

@@ -45,7 +45,10 @@ lib.mkIf config.sane.persist.enable
   # let sane.fs know about the mount
   sane.fs."${origin}".mount = {};
   # it also needs to know that the underlying device is an ordinary folder
-  sane.fs."${backing}" = sane-lib.fs.wantedDir;
+  sane.fs."${backing}" = sane-lib.fs.wanted {
+    dir.acl.user = config.sane.defaultUser;
+    # dir.acl.mode = "755";
+  };
   # in order for non-systemd `mount` to work, the mount point has to already be created, so make that a default target
   systemd.units = let
     originUnit = config.sane.fs."${origin}".generated.unit;