workflows: sbi-testing should be built under nightly

Signed-off-by: Zhouqi Jiang <luojia@hust.edu.cn>

.github/workflows/ci.yml

@@ -118,7 +118,7 @@ jobs:
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           target: ${{ matrix.TARGET }}
-          toolchain: stable
+          toolchain: nightly
       - uses: Swatinem/rust-cache@v2
       - name: Build
         run: |