Fix screenshots in GitHub Action when two jobs fail

The upload-artifact action does not support using the same artifact name
in different jobs (or in different matrix scenarios) since version 4,
which we started using in commit acfaada82. That meant that screenshots
were not uploaded correctly when two or more knapsack nodes failed.

.github/workflows/tests.yml

@@ -71,7 +71,7 @@ jobs:
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: screenshots
+          name: screenshots-${{ matrix.ci_node_index }}
           path: tmp/capybara/
   coveralls:
     permissions: