try to fix postgres install on ubuntu 20.04 for Python 3.6.

force update of apt inventory. Looks like postgresql 14 is supposed
to be installed, but apt install is trying v12.

Author: rouilj

.github/workflows/ci-test.yml

@@ -130,7 +130,7 @@ jobs:
 
       - name: Install postgres
         run: |
-          sudo apt-get install postgresql
+          sudo apt-get update && sudo apt-get install postgresql
           # Disable fsync for speed, don't care about data durability
           #   when testing
           sudo sed -i -e '$a\fsync = off' /etc/postgresql/*/*/postgresql.conf