ci: provision supported Python on runner

.gitea/workflows/ci.yml

@@ -13,26 +13,27 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      # The runner job image has a Debian-managed Python. Keep project packages
-      # isolated from it rather than modifying the runner's global environment.
-      - name: Set up a virtualenv
-        run: |
-          set -eu
-          python3 --version
-          python3 -m venv .venv
-          .venv/bin/python -m pip install --quiet --upgrade pip
+      # The runner job image only has Debian's Python 3.9 without ensurepip,
+      # while the project requires Python 3.10 or newer. Use the Actions tool
+      # cache rather than depending on the retired Debian 11 package mirrors.
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Check Python
+        run: python --version
 
       - name: Install
-        run: .venv/bin/python -m pip install --quiet -e ".[dev]"
+        run: python -m pip install --quiet -e ".[dev]"
 
       - name: Lint
-        run: .venv/bin/python -m ruff check .
+        run: python -m ruff check .
 
       - name: Test
-        run: .venv/bin/python -m pytest -q
+        run: python -m pytest -q
 
       - name: Check the CLI entry point
         run: |
           set -eu
-          .venv/bin/lean-bench --help > /dev/null
-          .venv/bin/lean-bench doctor --skip-llm --env /dev/null
+          lean-bench --help > /dev/null
+          lean-bench doctor --skip-llm --env /dev/null