fix: use venv for pip install to avoid externally-managed-environment

.forgejo/workflows/docs.yml

@@ -12,11 +12,15 @@ jobs:
           fetch-depth: 0
 
       - run: |
-          apt-get update && apt-get install -y python3 python3-pip
+          apt-get update && apt-get install -y python3 python3-pip python3-venv
+          python3 -m venv /tmp/venv
+          . /tmp/venv/bin/activate
           python3 --version
           pip3 install zensical
 
-      - run: zensical build --clean
+      - run: |
+          . /tmp/venv/bin/activate
+          zensical build --clean
 
       - name: Deploy to pages branch
         run: |