tox: make CI deterministic by disabling sitepackages

Using sitepackages pulled Fedora system Python packages and triggered
systemd %post scriptlets in CI containers. Switch default tox envs to
sitepackages=False.

Author: dichn

.github/workflows/main.yml

@@ -11,6 +11,8 @@ name: Run Tox tests
 jobs:
   tox_test:
     name: Tox test
+    env:
+      SYSTEMD_OFFLINE: 1
     steps:
     - name: Checkout kobo
       uses: actions/checkout@v2

tox.ini

@@ -12,8 +12,10 @@ deps =
     django3: Django~=3.2.0  # Django 3 LTS (EOL 4/2024)
     django4: Django~=4.2.0  # Django 4 LTS (EOL 4/2026)
     django5: Django~=5.0.0
-# for testing with python-rpm
-sitepackages = True
+
+# prevents pulling system Python + RPM packages,
+# which avoids systemd %post scriptlet failures in Fedora CI.
+sitepackages = False
 
 [testenv:py39-django3-cov]
 passenv = GITHUB_*