ci: replace ubuntu-latest with ubuntu-20.04

Now the ubuntu-latest points to Ubuntu 22.04 instead of 20.04. Due to
this two jobs in CI failed: "testing_linux (live/2.8)" and
"testing_linux (live/2.9)". See "GitHub Actions: Ubuntu-latest workflows
will use Ubuntu-22.04" [1].

1. https://github.blog/changelog/2022-11-09-github-actions-ubuntu-latest-workflows-will-use-ubuntu-22-04/

Author: ligurio

.github/workflows/testing.yml

@@ -60,7 +60,7 @@ jobs:
       # See https://github.com/packpack/packpack/issues/7
       DEBIAN_FRONTEND: noninteractive
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Export T_VERSION environment variable
         run: |