Keep a nightly for rustfmt

Co-authored-by: Tamo <tamo@meilisearch.com>

.github/workflows/rust.yml

@@ -74,7 +74,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: nightly
           override: true
           components: rustfmt
       - name: Cache dependencies