ci: Run pnpm dedupe --check.

New in pnpm 8.3.0, this replaces the yarn-deduplicate check that was
removed in commit 3a27b12a7d (#24731).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.github/workflows/zulip-ci.yml

@@ -195,6 +195,10 @@ jobs:
           source tools/ci/activate-venv
           ./tools/test-js-with-puppeteer
 
+      - name: Check pnpm dedupe
+        if: ${{ matrix.include_frontend_tests }}
+        run: pnpm dedupe --check
+
       - name: Check for untracked files
         run: |
           source tools/ci/activate-venv