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).

Author: ryoppippi

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

@@ -8,6 +8,11 @@ runs:
       with:
         github_access_token: ${{ github.token }}
 
+    - name: Cache Nix store
+      uses: nix-community/cache-nix-action@b426b118b6dc86d6952988d396aa7c6b09776d08 # v7
+      with:
+        primary-key: nix-${{ runner.os }}
+
     - name: Setup Cachix (numtide)
       uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
       with: