build: fix wrong directory for notification script (#19626)

The path to the Slack notification script seems to be incorrect.

Author: crisbeto

.circleci/config.yml

@@ -143,7 +143,7 @@ var_21: &slack_notify_on_failure
   run:
     name: "Notifying team about job failure"
     when: on_fail
-    command: ./scripts/notify-slack-job-failure.sh
+    command: ./scripts/circleci/notify-slack-job-failure.sh
 
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