use nix image for update

.github/workflows/update.yaml

@@ -5,19 +5,10 @@ on:
     - cron: "3 18 * * *"
 jobs:
   lockfile:
-    runs-on: ubuntu-latest
+    runs-on: nixpkgs/nix
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
-      - name: Install Nix
-        uses: https://github.com/cachix/install-nix-action@v30
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-          extra_nix_config: |
-            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-            substituters = https://cache.nixos.org/ https://attic.chir.rs/chir-rs/ https://hydra.chir.rs/
-            trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nixcache:8KKuGz95Pk4UJ5W/Ni+pN+v+LDTkMMFV4yrGmAYgkDg= chir-rs:rzK1Czm3RqBbZLnXYrLM6JyOhfr6Z/8lhACIPO/LNFQ=
-            experimental-features = nix-command flakes
       - name: update flake lock
         run: nix flake update
       - name: run update script