CI(replication-tests): fix notifications about replication-tests failures (#9950)

## Problem

`if: ${{ github.event.schedule }}` gets skipped if a previous step has
failed, but we want to run the step for both `success` and `failure`

## Summary of changes
- Add `!cancelled()` to notification step if-condition, to skip only
cancelled jobs

.github/workflows/benchmarking.yml

@@ -249,7 +249,7 @@ jobs:
 
     # Post both success and failure to the Slack channel
     - name: Post to a Slack channel
-      if: ${{ github.event.schedule }}
+      if: ${{ github.event.schedule && !cancelled() }}
       uses: slackapi/slack-github-action@v1
       with:
         channel-id: "C06T9AMNDQQ" # on-call-compute-staging-stream