Fix CI nix job by installing nix

Author: zah

.github/workflows/ci.yml

@@ -25,9 +25,12 @@ jobs:
 
   nix:
     runs-on: ubuntu-latest
-    container:
-      image: nixos/nix:latest
     steps:
       - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-24.05
+          extra_nix_config: |
+            experimental-features = nix-command flakes
       - name: Run tests via Nix
         run: nix develop -c just test