feat: cache nix packages in ci

.github/workflows/copilot-setup-steps.yml

@@ -35,5 +35,11 @@ jobs:
           extra_nix_config: |
             experimental-features = nix-command flakes
 
+      - name: Restore and save Nix store cache
+        uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+
       - name: Install flake dependencies
         run: nix develop --accept-flake-config -c true