perf(ci): add pnpm store cache for faster dependency install

Cache pnpm store directory to avoid re-downloading packages.
The cache key is based on OS and pnpm-lock.yaml hash.

Author: ryoppippi

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

@@ -34,6 +34,21 @@ runs:
         done
         nix profile install --inputs-from . $packages
 
+    - name: Get pnpm store directory
+      if: inputs.skip-pnpm-install != 'true'
+      id: pnpm-cache
+      shell: bash
+      run: echo "STORE_PATH=$(pnpm store path)" >> $GITHUB_OUTPUT
+
+    - name: Restore pnpm cache
+      if: inputs.skip-pnpm-install != 'true'
+      uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4.2.3
+      with:
+        path: ${{ steps.pnpm-cache.outputs.STORE_PATH }}
+        key: pnpm-${{ runner.os }}-${{ hashFiles('pnpm-lock.yaml') }}
+        restore-keys: |
+          pnpm-${{ runner.os }}-
+
     - name: Install pnpm dependencies
       if: inputs.skip-pnpm-install != 'true'
       shell: bash