add rust beta to zebra-chain building job

.github/workflows/ci.yml

@@ -52,14 +52,18 @@ jobs:
           args: --verbose --manifest-path zebrad/Cargo.toml sync_large_checkpoints_ -- --ignored
 
   build-chain-no-features:
-    name: Build zebra-chain w/o features on ubuntu-latest
+    name: Build zebra-chain w/o features (+${{ matrix.rust }}) on ubuntu-latest
     timeout-minutes: 30
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-latest    
+    strategy:
+      matrix:
+        rust: [stable, beta]
+    
     steps:
       - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: ${{ matrix.rust }}
           override: true
       - name: cargo fetch
         uses: actions-rs/cargo@v1