github: Use nightly channel for rustfmt.

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: nightly
           override: true
       - run: |
           rustup component add rustfmt