CI: test cache with other nix install action

Author: algorithmiker

.github/workflows/test.yml

@@ -10,7 +10,9 @@ jobs:
       id-token: write
     steps:
     - uses: actions/checkout@v4
-    - uses: DeterminateSystems/nix-installer-action@main
+    - uses: cachix/install-nix-action@v31
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
     - uses: DeterminateSystems/magic-nix-cache-action@main
     - run: nix build -v -L .#entrace
     - run: nix build -v -L .#entrace_core