fix: use nightly for tests

Author: lgalabru

.github/workflows/ci.yaml

@@ -69,7 +69,7 @@ jobs:
       - name: Install Rust toolchain
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: nightly
           profile: minimal
           components: llvm-tools-preview
           override: true