Use rust-components-nightly action for checking formatting

Ensures that the test doesn't fail if the lastest nightly has no rustfmt.

Author: phil-opp

.github/workflows/build.yml

@@ -124,5 +124,15 @@ 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