Coax the CI build into a working state

 - The cargo bench invocation wants rust 1.81
 - setup-rust-toolchain doesn't install rustfmt unless we ask

Author: dholroyd

.github/workflows/rust.yml

@@ -16,10 +16,14 @@ jobs:
 
     - name: Install toolchain
       uses: actions-rust-lang/setup-rust-toolchain@v1.15.1
+      with:
+        components: rustfmt
 
     - name: Check formatting
       run: cargo fmt --check
+
     - name: Build
       run: cargo build --verbose --all-targets
+
     - name: Run tests
       run: cargo test --verbose

rust-toolchain.toml

@@ -1,2 +1,2 @@
 [toolchain]
-channel = "1.79"
+channel = "1.81"