github: use ubuntu 20.04 runner for postgres

It seems like Ubuntu 22.04 (current latest) terminates build jobs
because of high CPU usage sometimes (see
actions/runner-images#6680).

We attempt to fix this by using the older runner.

.github/workflows/main.yaml

@@ -208,7 +208,7 @@ jobs:
   ########################
   integration-test-postgres:
     name: run itests postgres
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: git checkout
         uses: actions/checkout@v3