chore(ci): switch to cache-nix-action for Nix store caching (#87)

* chore(ci): switch to cache-nix-action for Nix store caching

Replace cachix-action (numtide) with nix-community/cache-nix-action.
This uses GitHub Actions cache to store /nix/store directly, providing
faster CI builds without requiring external Cachix configuration.

Changes:
- Update install-nix-action to v31.9.0
- Replace cachix-action with cache-nix-action v7
- Use OS-based cache key for broader cache hits

* test cache

Author: ryoppippi

.github/actions/setup-nix/action.yaml

@@ -1,18 +1,17 @@
 name: "Setup Nix"
-description: "Install Nix and configure Cachix"
+description: "Install Nix and configure cache"
 runs:
   using: "composite"
   steps:
     - name: Install Nix
-      uses: cachix/install-nix-action@0b0e072294b088b73964f1d72dfdac0951439dbd # v31.8.4
+      uses: cachix/install-nix-action@4e002c8ec80594ecd40e759629461e26c8abed15 # v31.9.0
       with:
         github_access_token: ${{ github.token }}
 
-    - name: Setup Cachix (numtide)
-      uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
+    - name: Cache Nix store
+      uses: nix-community/cache-nix-action@b426b118b6dc86d6952988d396aa7c6b09776d08 # v7
       with:
-        name: numtide
-        authToken: ""
+        primary-key: nix-${{ runner.os }}
 
     - name: Load Nix development environment
       shell: bash