Use Rust nightly in CI

Needed for rust-lang/rust#93050

Author: csgordon

.github/workflows/rust.yml

@@ -16,6 +16,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - name: Switch to nightly
+      run: rustup default nightly
     - name: Build
       run: cargo build --verbose
     - name: Run tests