CI: fail if lockfile is out of date

Author: rmburg

.github/workflows/rust.yml

@@ -24,6 +24,6 @@ jobs:
     - name: Rustfmt Check
       uses: actions-rust-lang/rustfmt@v1
     - name: Build
-      run: cargo build --verbose
+      run: cargo build --verbose --locked
     - name: Run tests
       run: cargo test --verbose