perf(ci): add nix-community/cache-nix-action for store caching

Cache Nix store paths to speed up subsequent CI runs.
The cache key includes OS, flake.lock hash, and tools list.

Features:
- Restores from cache before installing packages
- Saves updated store after job completion
- Garbage collection keeps cache under 1GB
- Falls back to partial cache matches

Author: ryoppippi

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

@@ -15,6 +15,16 @@ runs:
     - name: Install Nix
       uses: nixbuild/nix-quick-install-action@5bb6a3b3abe66fd09bbf250dce8ada94f856a703 # v30
 
+    - name: Restore Nix cache
+      uses: nix-community/cache-nix-action@135667ec418502fa5a3598af6fb9eb733888ce6a # v6
+      with:
+        primary-key: nix-${{ runner.os }}-${{ hashFiles('flake.lock') }}-${{ inputs.tools }}
+        restore-prefixes-first-match: |
+          nix-${{ runner.os }}-${{ hashFiles('flake.lock') }}-
+          nix-${{ runner.os }}-
+        gc-max-store-size-linux: 1G
+        gc-max-store-size-macos: 1G
+
     - name: Install tools from nixpkgs
       shell: bash
       run: |