[ci] fix install of OS packages in CI workflows

We should be using "apt-get update" to update package lists before
installing any new packages. This has never mattered before, but
recently the installation of various packages started to fail with
404 errors, which are resolved by updating the package lists first.

.github/workflows/docs.yml

@@ -15,8 +15,10 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.9
-      - name: Install system dependencies
-        run: sudo apt-get install -y libkrb5-dev
+      - name: Install OS packages
+        run: |
+          sudo apt-get -y update
+          sudo apt-get install -y libkrb5-dev
       - name: Install Tox
         run: pip install tox
       - name: Run Tox