add workaround for 404 error when installing packages in CI workflow …

…for testing Apptainer integration

.github/workflows/container_tests_apptainer.yml

@@ -29,10 +29,18 @@ jobs:
     - name: install OS & Python packages
       run: |
         # for building CentOS 7 container images
-        sudo apt-get install rpm
-        sudo apt-get install dnf
+        APT_PKGS="rpm dnf"
         # for modules tool
-        sudo apt-get install lua5.2 liblua5.2-dev lua-filesystem lua-posix tcl tcl-dev
+        APT_PKGS="lua5.2 liblua5.2-dev lua-filesystem lua-posix tcl tcl-dev"
+
+        # Avoid apt-get update, as we don't really need it,
+        # and it does more harm than good (it's fairly expensive, and it results in flaky test runs)
+        if ! sudo apt-get install $APT_PKGS; then
+          # Try to update cache, then try again to resolve 404s of old packages
+          sudo apt-get update -yqq || true
+          sudo apt-get install $APT_PKGS
+        fi
+
         # fix for lua-posix packaging issue, see https://bugs.launchpad.net/ubuntu/+source/lua-posix/+bug/1752082
         # needed for Ubuntu 18.04, but not for Ubuntu 20.04, so skipping symlinking if posix.so already exists
         if [ ! -e /usr/lib/x86_64-linux-gnu/lua/5.2/posix.so ] ; then