Use latest nightly with rustfmt for formatting check (#679)

Don't fail if current nightly has no rustfmt

.github/workflows/build-code.yml

@@ -66,5 +66,15 @@ jobs:
     timeout-minutes: 2
     steps:
     - uses: actions/checkout@v1
+    - name: "Find latest Nightly with rustfmt"
+      id: component
+      uses: actions-rs/components-nightly@v1
+      with:
+        component: rustfmt
+    - name: "Add a rustup override for rustfmt nightly"
+      uses: actions-rs/toolchain@v1
+      with:
+          toolchain: ${{ steps.component.outputs.toolchain }}
+          override: true
     - run: rustup component add rustfmt
     - run: cargo fmt -- --check