sig-scalability: keep more detailed JUnit for scheduler_perf job

Instead of combining all tests under their package, each test is
listed separately. This makes it more visible how long each test runs.

This mirrors the corresponding change made earlier in
#33809

config/jobs/kubernetes/sig-scalability/sig-scalability-periodic-jobs.yaml

@@ -761,6 +761,10 @@ periodics:
       # We have to explicitly disable --short flag because the script enables by default.
       - name: SHORT
         value: --short=false
+      # Capture success/failure and duration for each individual test case instead
+      # of reducing everything at the package level.
+      - name: KUBE_PRUNE_JUNIT_TESTS
+        value: "false"
       # We need to constraint compute resources so all the tests
       # finish approximately at the same time. More compute power
       # can increase scheduling throughput and make consequent results