Switch to nightly for target tests

Author: alexcrichton

.github/workflows/main.yml

@@ -189,7 +189,7 @@ jobs:
       with:
         submodules: true
     - name: Install Rust
-      run: rustup update stable && rustup default stable
+      run: rustup update nightly && rustup default nightly
     - run: rustup target add ${{ matrix.target }}
     - run: cargo build --target ${{ matrix.target }}