ci: add workaround for failing container jobs

Node.js 20 is not supported in older containers like ubuntu:18.04,
but GitHub Actions nowawadays forces the use of Node.js 20, even
if the action itself still specified Node.js 16 as the version to
use. The result is that e.g. `actions/checkout@v3` fails in the
containers, although it should not (because v3 is still specified
Node.js 16).

See <https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/>
for more information on that forced Node.js version. Fortunately,
there still is a workaround to restore the old behaviour, and
that is to set a environment variable to allow older Node.js to
be used in those actions, and that is what this change does.

.github/workflows/ci.yml

@@ -11,6 +11,7 @@ on:
 env:
   LIBRARY: gil
   UBSAN_OPTIONS: print_stacktrace=1
+  ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
 
 jobs:
   posix: