ci: Fix matrix definition for tests job.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.github/workflows/zulip-ci.yml

@@ -27,8 +27,6 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        include_documentation_tests: [false]
-        include_frontend_tests: [false]
         include:
           # Base images are built using `tools/ci/Dockerfile.prod.template`.
           # The comments at the top explain how to build and upload these images.
@@ -36,16 +34,20 @@ jobs:
           - docker_image: zulip/ci:focal
             name: Ubuntu 20.04 (Python 3.8, backend + frontend)
             os: focal
+            include_documentation_tests: false
             include_frontend_tests: true
           # Debian 11 ships with Python 3.9.2.
           - docker_image: zulip/ci:bullseye
             name: Debian 11 (Python 3.9, backend + documentation)
             os: bullseye
             include_documentation_tests: true
+            include_frontend_tests: false
           # Ubuntu 22.04 ships with Python 3.10.4.
           - docker_image: zulip/ci:jammy
             name: Ubuntu 22.04 (Python 3.10, backend)
             os: jammy
+            include_documentation_tests: false
+            include_frontend_tests: false
 
     runs-on: ubuntu-latest
     name: ${{ matrix.name }}