GitHub Actions: Pass --component rustfmt in formatting job

Author: phil-opp

.github/workflows/build.yml

@@ -61,5 +61,5 @@ jobs:
     timeout-minutes: 2
     steps:
     - uses: actions/checkout@v1
-    - run: rustup install nightly
+    - run: rustup install nightly -c rustfmt
     - run: cargo +nightly fmt -- --check