Simplify CI workflow - use available python3 and avoid apt-get if not needed

.gitea/workflows/ci.yml

@@ -12,18 +12,18 @@ jobs:
     steps:
       - name: Checkout code
         run: |
-          mkdir -p ${{ github.repository }}
+          git clone https://gitea.com/${{ github.repository }} ${{ github.workspace }}
-          git clone https://gitea.com/${{ github.repository }} .
+          cd ${{ github.workspace }}
           git checkout ${{ github.sha }}
       - name: Set up Python
         run: |
-          apt-get update
+          which python3 || (apt-get update && apt-get install -y python3 python3-pip)
-          apt-get install -y python3 python3-pip
+          python3 --version
-          python3 -m pip install --upgrade pip
       - name: Install dependencies
         run: |
-          pip3 install -e .
+          python3 -m pip install --upgrade pip
-          pip3 install pytest
+          python3 -m pip install -e .
+          python3 -m pip install pytest
       - name: Run tests
         run: |
           python3 -m pytest tests/ -v