Fix checkout support in CI for Ubuntu 18.04

The combination of us using the past-EoL Ubuntu 18.04 and Microsoft
being a bit gung-ho in their changes to GitHub Actions means that
Node16, and by extension checkouts@v3 is no longer supported. Since
Ubuntu 18.04 doesn't support the newer Node20, we are stuck. Thankfully,
we can set the `ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION` environment
variable which is project to keep the CI running until they remove
support for Node16 fully.

.github/workflows/builds.yml

@@ -10,6 +10,10 @@ name: Build Tests
 # Perform the builds on every push and pull request.
 on: [ push, pull_request ]
 
+# Hack to continue using checkout (for now) on Ubuntu 18.04.
+# see: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/
+env:
+  - ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}