ci: fix toolchain install

Author: ValuedMammal

.github/workflows/rust.yml

@@ -32,6 +32,7 @@ jobs:
       - name: Install Rust
         uses: dtolnay/rust-toolchain@v1
         with:
+            toolchain: stable
             components: clippy, rustfmt
       - name: Rust fmt
         run: cargo fmt --all -- --check