fix #315, and add cargo check in GitHub CI with zk feature disabled (#316)

.github/workflows/build.yml

@@ -20,6 +20,8 @@ jobs:
         toolchain: stable
     - name: Build default
       run: cargo build
+    - name: Build non-zk # check without the zk feature enabled
+      run: cargo build --no-default-features --features backend_plonky2
     - name: Build metrics
       run: cargo build --features metrics
     - name: Build time