feat: add rocksdb updater nix app

nix/default.nix

@@ -5,5 +5,6 @@
     ./packages
     ./devshell.nix
     ./fmt.nix
+    ./rocksdb-updater.nix
   ];
 }

nix/rocksdb-updater.nix

@@ -0,0 +1,14 @@
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      apps.update-rocksdb = {
+        type = "app";
+        program = pkgs.writeShellApplication {
+          name = "update-rocksdb";
+          runtimeInputs = [ pkgs.nix-update ];
+          text = "nix-update rocksdb -F --version branch";
+        };
+      };
+    };
+}