fix: move concurrency parameters to substep (#116461)

Concurrency parameters were not in the correct place which made concurrency settings not in effect.
Moved it to to performance.sh step which also has parallelism in the same level.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

.buildkite/pipelines/performance/nightly.yml

@@ -24,8 +24,6 @@ steps:
     agents:
       queue: ci-group-6
     depends_on: build
-    concurrency: 50
-    concurrency_group: 'performance-test-group'
 
   - wait: ~
     continue_on_failure: true

.buildkite/scripts/steps/functional/performance.sh

@@ -14,6 +14,8 @@ cat << EOF | buildkite-agent pipeline upload
 steps:
   - command: .buildkite/scripts/steps/functional/performance_sub.sh
     parallelism: "$ITERATION_COUNT"
+    concurrency: 20
+    concurrency_group: 'performance-test-group'
 EOF