[.github] codecov.yml: Explicitly specify junit.xml.

Otherwise, it ends up uploading TinyXML2 test XMLs in addition to
junit.xml.

.github/workflows/codecov.yml

@@ -28,3 +28,4 @@ jobs:
         uses: codecov/test-results-action@v1
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
+          files: ./junit.xml