fix codecov upload (closes #381)

The codecov action can generate a usable report itself, but it needs the
coverage package installed and accessible. This is not the case anymore,
since it's now only installed in the hatch test environment. So we
generate the report ourselves.

Author: ilia-kats

{{cookiecutter.project_name}}/.github/workflows/test.yaml

@@ -55,5 +55,7 @@ jobs:
           PLATFORM: ${{ matrix.os }}
           DISPLAY: :42
         run: uvx hatch test --cover --python ${{ matrix.python }}
+      - name: generate coverage report
+        run: uvx coverage xml
       - name: Upload coverage
         uses: codecov/codecov-action@v4