ci: drop actions/setup-python from the remaining workflows

Applies the same fix as test/lint to every workflow that still used
actions/setup-python, which the old act_runner engine mishandles:

- update-badges: was broken identically to lint — setup-python + pip
  install hit the image's externally-managed system Python. Drop
  setup-python, install with --break-system-packages, and use `python3`
  (not bare `python`) for the coverage steps.
- canaries, prd-number: no pip install, so not failing, but they carried
  the same fragile (and network-heavy) setup-python for stdlib-only work.
  Removed — the image's system Python 3.12 runs them directly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

.gitea/workflows/canaries.yml

@@ -22,10 +22,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up Python
-        uses: actions/setup-python@v5
-        with:
-          python-version: "3.12"
-
+      # No actions/setup-python: canaries are stdlib unittest on the image's
+      # system Python 3.12 (older act_runner mishandles setup-python's PATH).
       - name: Run canaries
         run: python3 -m unittest discover -t . -s tests/canaries -v