GHA: update/fix codecov config

Author: lafrech

.github/workflows/build-release.yaml

@@ -25,7 +25,9 @@ jobs:
           python -m pip install tox
           python -m tox -e${{ matrix.tox }}
       - name: Upload coverage
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
+        with:
+          token: ${{ secrets.codecov_token }}
   build:
     name: Build package
     runs-on: ubuntu-latest