CI: Fetch apt package lists before trying to install Pulsar

New runner image doesn't have any package lists pre-cached, maybe?

---

Passing (older image)

  Image: ubuntu-20.04
  Version: 20241215.1.0
  Included Software: https://github.com/actions/runner-images/blob/ubuntu20/20241215.1/images/ubuntu/Ubuntu2004-Readme.md
  Image Release: https://github.com/actions/runner-images/releases/tag/ubuntu20%2F20241215.1

Vs failing (newer image)

  Image: ubuntu-20.04
  Version: 20250105.1.0
  Included Software: https://github.com/actions/runner-images/blob/ubuntu20/20250105.1/images/ubuntu/Ubuntu2004-Readme.md
  Image Release: https://github.com/actions/runner-images/releases/tag/ubuntu20%2F20250105.1

.github/workflows/package-tests-linux.yml

@@ -162,6 +162,9 @@ jobs:
           - package: "language-yaml"
 
     steps:
+    - name: Fetch/update package lists for apt package repos
+      run: sudo apt-get update
+
     - name: Checkout the latest code
       uses: actions/checkout@v4
     - name: Setup NodeJS