Use nightly rustfmt on CI

Author: alcuadrado

.github/workflows/hardhat-core-ci.yml

@@ -282,18 +282,26 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - name: Install nightly rustfmt
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          profile: minimal
+          components: rustfmt
+
       - name: Install stable toolchain
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
           override: true
-          components: clippy, rustfmt
+          components: clippy
 
       - uses: Swatinem/rust-cache@v2
 
       - name: Run cargo fmt
         uses: actions-rs/cargo@v1
         with:
+          toolchain: nightly
           command: fmt
           args: --all --check