Set ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION for container jobs

Node 20 isn't supported in containers and GitHub actions forces actions
to use Node 20 even when the action is specified to use Node 16.
See https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default

This causes e.g. `actions/checkout@v3` to fail since that change.
A workaround is to set `$ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION` to
prevent this change (for now).

.github/workflows/ci.yml

@@ -143,6 +143,7 @@ jobs:
             if [[ "${{matrix.container}}" == "ubuntu:16.04" ]] || [[ "${{matrix.container}}" == "ubuntu:18.04" ]]; then
               # Ubuntu 16/18 can't run Node 20, so stick to older actions: https://github.com/actions/checkout/issues/1590
               echo "GHA_USE_NODE_20=false" >> $GITHUB_ENV
+              echo "ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true" >> $GITHUB_ENV
             else
               echo "GHA_USE_NODE_20=true" >> $GITHUB_ENV
             fi