Add a CI check that Cargo.lock is up to date (#35111)

Should catch cases like #33667 where a dependency was bumped in
Cargo.toml without a corresponding lockfile update. (This can happen
when committing from outside of Zed or if rust-analyzer isn't running.)
Passing `--frozen --workspace` should ensure this doesn't fail just
because there's a newer patch version of some third-party dependency,
and prevent it from taking long.

We only need to run this on macOS because Cargo.lock is
platform-independent.

Release Notes:

- N/A

.github/workflows/ci.yml

@@ -269,6 +269,10 @@ jobs:
           mkdir -p ./../.cargo
           cp ./.cargo/ci-config.toml ./../.cargo/config.toml
 
+      - name: Check that Cargo.lock is up to date
+        run: |
+          cargo update --frozen --workspace
+
       - name: cargo clippy
         run: ./script/clippy