ci: use uv to provision Python 3.12, fixing zensical build on py3.9 runner

.forgejo/workflows/docs.yml

@@ -5,26 +5,30 @@ on:
       - main
 jobs:
   deploy:
-    runs-on:
-      container:
-        image: python:3.12
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
-      - run: python3 --version
+      - name: Install uv
+        run: |
+          curl -LsSf https://astral.sh/uv/install.sh | sh
+          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
 
-      - run: |
-          apt-get update
-          apt-get install -y python3-venv python3-pip
+      - name: Install Python 3.12 via uv
+        run: |
+          export PATH="$HOME/.local/bin:$PATH"
+          uv python install 3.12
 
-      - run: |
-          python3 -m venv /tmp/venv
+      - name: Build site
+        run: |
+          export PATH="$HOME/.local/bin:$PATH"
+          uv venv --python 3.12 /tmp/venv
           . /tmp/venv/bin/activate
-          pip install --upgrade pip
-          pip install zensical
-          pip install -e .
+          python --version
+          uv pip install zensical
+          uv pip install -e .
           zensical build --clean
 
       - name: Deploy to pages branch