Bump GitHub Actions to Node.js 24 compatible versions

- actions/checkout v4 -> v6
- cachix/install-nix-action v27 -> v31 (also patches CVE-2026-39860)
- stefanzweifel/git-auto-commit-action v5 -> v7

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/update.yml

@@ -13,10 +13,10 @@ jobs:
  
     steps:
       - name: Checkout repo
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Install Nix
-        uses: cachix/install-nix-action@v27
+        uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
           extra_nix_config: |
@@ -26,7 +26,7 @@ jobs:
         run: nix flake update
  
       - name: Commit updated flake.lock
-        uses: stefanzweifel/git-auto-commit-action@v5
+        uses: stefanzweifel/git-auto-commit-action@v7
         with:
           commit_message: "flake: update inputs"
           file_pattern: flake.lock