chore(ci): check rustfmt with nightly toolchain

Some of our rustfmt rules aren't stable yet. Thus our rustfmt CI check should run on nightly.

Pull-Request: libp2p#5743.

Author: elenaf9

.github/workflows/ci.yml

@@ -317,7 +317,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@nightly
         with:
           components: rustfmt