Fix CI workflow: use find

seems that galaxyproject#7293
introduced some problem if there is only one artifact

Author: bernt-matthias

.github/workflows/pr.yaml

@@ -354,7 +354,8 @@ jobs:
         mode: check
     - name: Check if all test chunks succeeded
       run: |
-        NFILES=$(ls artifacts/ | grep "Tool test output" | wc -l)
+        ls artifacts/
+        NFILES=$(find artifacts/ -name "tool_test_output.json" | wc -l)
         if [[ "${{ needs.setup.outputs.chunk-count }}" != "$NFILES" ]]; then
           exit 1
         fi