chore(ci): add cache for Nix workflow (#1210)

* chore(ci): Add cache for Nix workflow

* align lockfile with other projects

* Include both flake.lock and cargo.lock in hash

.github/workflows/test.yml

@@ -30,6 +30,32 @@ jobs:
           nix_path: nixpkgs=channel:nixos-22.11
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
 
+      - uses: cachix/cachix-action@v12
+        with:
+          name: barretenberg
+
+      - name: Restore nix store cache
+        id: nix-store-cache
+        uses: actions/cache@v3
+        with:
+          path: /tmp/nix-cache
+          key: ${{ runner.os }}-flake-${{ hashFiles('*.lock') }}
+
+      # Based on https://github.com/marigold-dev/deku/blob/b5016f0cf4bf6ac48db9111b70dd7fb49b969dfd/.github/workflows/build.yml#L26
+      - name: Copy cache into nix store
+        if: steps.nix-store-cache.outputs.cache-hit == 'true'
+        # We don't check the signature because we're the one that created the cache
+        run: |
+          for narinfo in /tmp/nix-cache/*.narinfo; do
+            path=$(head -n 1 "$narinfo" | awk '{print $2}')
+            nix copy --no-check-sigs --from "file:///tmp/nix-cache" "$path"
+          done
+
       - name: Run `nix flake check`
         run: |
-          nix flake check
+          nix flake check -L
+
+      - name: Export cache from nix store
+        if: steps.nix-store-cache.outputs.cache-hit != 'true'
+        run: |
+          nix copy --to "file:///tmp/nix-cache?compression=zstd&parallel-compression=true" .#cargo-artifacts

flake.nix

@@ -165,6 +165,9 @@
 
       packages.default = noir;
 
+      # We expose the `cargo-artifacts` derivation so we can cache our cargo dependencies in CI
+      packages.cargo-artifacts = cargoArtifacts;
+
       # TODO(#1197): Look into installable apps with Nix flakes
       # apps.default = flake-utils.lib.mkApp { drv = nargo; };