workflows/eval: Improve debuggabilitiy

Sometimes successful workflows trigger that condition,
see https://github.com/NixOS/nixpkgs/actions/runs/12278493443/job/34260660227:

    Comparing against "https://github.com/NixOS/nixpkgs/actions/runs/12278495895"
    Workflow not done, waiting 10 seconds before checking again
    Workflow was not successful, cannot make comparison

Even though

    $ gh api /repos/NixOS/nixpkgs/actions/runs/12278495895 --jq .conclusion
    success

Let's print the conclusion when it's not "success" to debug this.

.github/workflows/eval.yml

@@ -179,7 +179,7 @@ jobs:
           done
 
           if [[ "$conclusion" != "success" ]]; then
-            echo "Workflow was not successful, cannot make comparison"
+            echo "Workflow was not successful (conclusion: $conclusion), cannot make comparison"
             exit 0
           fi