Upload codecov only on ubuntu

Author: mjwen

.github/workflows/testing.yml

@@ -65,7 +65,7 @@ jobs:
           pytest --cov=kliff --cov-report=xml
 
       - uses: codecov/codecov-action@v4
-        if: matrix.python-version == '3.10' && github.repository == 'openkim/kliff'
+        if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.10' && github.repository == 'openkim/kliff'
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
           file: ./coverage.xml