Run temp functional test workflow every 20 mins (radius-project#7368)

# Description

This is to run functional test with retry workflow every 20 minutes.

## Type of change

- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
(issue link optional).

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Signed-off-by: Young Bu Park <[email protected]>

.github/workflows/retry-functional-test.yaml

@@ -23,8 +23,8 @@ permissions:
 
 on:
   schedule:
-    # Run every hour
-    - cron: "0 1-23 * * *"
+    # Run every 20 minutes
+    - cron: "*/20 * * * *"
 
 env:
   # Go version