Backport #39818 to stable-2.5 (#39824)

* Update vars for loop_control on each loop iteration (fixes #38899) (#39818)

(cherry picked from commit 01e8139d74)

* Changelog snippet for cherry-pick of #39818

changelogs/fragments/39818-loop_control_task_vars.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - loop_control - update template vars for loop_control fields on each loop iteration (https://github.com/ansible/ansible/pull/39818).

lib/ansible/executor/task_executor.py

@@ -302,6 +302,9 @@ class TaskExecutor:
             if index_var:
                 task_vars[index_var] = item_index
 
+            # Update template vars to reflect current loop iteration
+            templar.set_available_variables(task_vars)
+
             # pause between loop iterations
             if loop_pause and ran_once:
                 try: