Better delimit the coverage files to upload

The previous setting was not working and then the action
was defaulting to "auto-search" mode, causing the whole
workspace (moodle, plugins,...) to be inspected and leading
to the upload of some false-positives like coverage.php files.

Now, only files under the coverage dir will be considered.

.github/workflows/ci.yml

@@ -130,6 +130,6 @@ jobs:
         uses: codecov/codecov-action@v4
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-          files: ./coverage/bats/cobertura.xml
+          directory: ${{ github.workspace }}/coverage
           verbose: true