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

@@ -1,10 +1,10 @@
 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 }}
 
@@ -13,12 +13,6 @@ runs:
       with:
         primary-key: nix-${{ runner.os }}
 
-    - name: Setup Cachix (numtide)
-      uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
-      with:
-        name: numtide
-        authToken: ''
-
     - name: Load Nix development environment
       shell: bash
       run: nix develop --command true