production_suite: Add debian production install to production suite.

We support Debian as an OS for setting up the Zulip server. But the CI
does not run on pull request to test the setting up of the server on
Debian. Hence, add the check to CI.

.github/workflows/production-suite.yml

@@ -120,6 +120,11 @@ jobs:
             is_focal: true
             os: focal
 
+          - docker_image: ligmitz/actions:buster
+            name: Buster production install
+            is_buster: true
+            os: buster
+
     name: ${{ matrix.name  }}
     container: ${{ matrix.docker_image }}
     runs-on: ubuntu-latest