feat(ci): add dedicated job for ensuring the lockfile is up-to-date

Instead of running with `--locked` which would require us to pick a CI job that does this, we can add a dedicated job that only has as its responsibility to ensure that `Cargo.lock` is up-to-date.

Fixes #3547.

Pull-Request: #3587.

.github/workflows/ci.yml

@@ -288,3 +288,10 @@ jobs:
 
             echo $git_status
             test -z "$git_status"
+
+  ensure-lockfile-uptodate:
+    name: Ensure that `Cargo.lock` is up-to-date
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: cargo metadata --frozen --format-version=1 > /dev/null