Add --locked flag to CI test run.

This is what we wanted in the first place. It has the added benefit of
causing an error if / when the Cargo.lock file gets updated and it
shouldn't.

.github/workflows/ci.yaml

@@ -23,4 +23,4 @@ jobs:
       - uses: dtolnay/rust-toolchain@nightly
       - run: sudo apt-get update
       - run: sudo apt-get install -y pkg-config libudev-dev
-      - run: cargo test --all-features
+      - run: cargo test --all-features --locked