ci(nix): add Nix store caching to setup-nix action (#268)

* ci(nix): add Nix store caching to setup-nix action

Add nix-community/cache-nix-action to cache the Nix store between CI
runs. This speeds up subsequent builds by avoiding redundant downloads
and builds of Nix dependencies.

The cache key is based on the runner OS (e.g. nix-Linux, nix-macOS).

* ci(nix): replace cachix with cache-nix-action

Remove cachix-action in favour of nix-community/cache-nix-action for
caching the Nix store. Also update install-nix-action to v31.9.0.

This simplifies the caching setup and aligns with stackone-ai-python.

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