ci(lint): drop actions/setup-python; install into the container's system Python

Same fix as the test workflow: the old act_runner engine mishandles
actions/setup-python's PATH, so `pip install` hit the image's
externally-managed system Python and failed with
"externally-managed-environment" on the "Install dev dependencies" step.

The runner image already ships Python 3.12 and the job container is
ephemeral, so drop setup-python and install straight into system Python
with --break-system-packages. pylint/pyright console scripts land on
/usr/local/bin (on PATH), so the lint steps still resolve. Also drops the
now-pointless `pip install --upgrade pip`.

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

.gitea/workflows/lint.yml

@@ -13,15 +13,14 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - name: Set up Python
+      # No actions/setup-python: the runner image already ships Python 3.12,
-        uses: actions/setup-python@v4
+      # and older act_runner engines mishandle setup-python's PATH. Install
-        with:
+      # into the ephemeral job container's system Python — the pylint/pyright
-          python-version: "3.12"
+      # console scripts land on /usr/local/bin (on PATH) so the steps below
-
+      # still resolve. --break-system-packages is safe: the container is
+      # disposable.
       - name: Install dev dependencies
-        run: |
+        run: python3 -m pip install --break-system-packages -r requirements-dev.txt
-          python -m pip install --upgrade pip
-          pip install -r requirements-dev.txt
 
       - name: Run pylint
         run: |