fix CI: use nightly rust for int_roundings feature

Author: masoud@anyscale.com

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@nightly
         with:
           components: rustfmt, clippy