Update CI

Switch to Rust nightly

Author: ccgauche

.github/workflows/rust.yml

@@ -17,6 +17,6 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Build
-      run: cargo build --verbose
+      run: cargo +nightly build --verbose
     - name: Run tests
-      run: cargo test --verbose
+      run: cargo +nightly test --verbose