CI: Use macOS 14 runner, pin the nix channel

.github/workflows/build-artifacts.yml

@@ -7,16 +7,20 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-22.04
+    runs-on: macos-14
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
       - name: Install Nix
         uses: cachix/install-nix-action@v26
         with:
+          nix_path: nixpkgs=channel:nixos-23.11
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Check Nix Flake
+        run: nix flake check
+
       - name: Build Anki deck and PDF with Nix
         run: nix develop --command bash -c 'make cards.apkg cards.pdf'