CI: fix coverage job

.github/workflows/main.yml

@@ -61,8 +61,6 @@ jobs:
     # only run on pushes to branches in the sphinx-doc/sphinx repo
     if: github.repository_owner == 'sphinx-doc' && github.event_name == 'push'
     runs-on: ubuntu-latest
-    env:
-      PYTEST_ADDOPTS: "--cov ./ --cov-append --cov-config setup.cfg"
 
     steps:
     - uses: actions/checkout@v3
@@ -78,10 +76,12 @@ jobs:
       run: sudo apt-get install graphviz
 
     - name: Install dependencies
-      run: python -m pip install -U tox pip codecov pytest-cov
+      run: python -m pip install -U pip tox pytest-cov
 
     - name: Run Tox
-      run: tox -e py -- -vv
+      run: tox --sitepackages -e py -- -vv
+      env:
+        PYTEST_ADDOPTS: "--cov ./ --cov-append --cov-config setup.cfg"
 
     - name: codecov
       uses: codecov/codecov-action@v3