CI: Only attempt to upload coverage reports with COVERAGE: true

Otherwise we get a flurry of warning in GitHub.

Author: Geod24

.github/workflows/main.yml

@@ -133,4 +133,5 @@ jobs:
       shell: bash
 
     - name: Codecov
+      if: matrix.do_test && runner.os != 'Windows'
       uses: codecov/codecov-action@v4