GHA: Fix CI on old Ubuntu

GitHub actions/checkout@v3 is no longer working and v4 is incompatible
with older Linux versions due to actions/checkout#1590.

Solution taken from
arenadata/gpdb@4b34d83

.github/workflows/ci.yml

@@ -3,6 +3,10 @@ name: CI
 
 on: [ push, pull_request ]
 
+env:
+   # workaround required for checkout@v3, https://github.com/actions/checkout/issues/1590
+   ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
+
 jobs:
   build:
     runs-on: ubuntu-latest