Build/Test Tools: Correct the usage of the conclusion of the previous run within the Slack notifications workflow.

Prior to r59679 this value was echoed to GitHub output which meant that its surrounding double quotes lost their significance. Now this value is used directly in the job output it needs to be treated as a plain string.

This concludes the conclusion confusion.

See #82221


git-svn-id: https://develop.svn.wordpress.org/trunk@59693 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/slack-notifications.yml

@@ -64,6 +64,7 @@ jobs:
         with:
           retries: 2
           retry-exempt-status-codes: 418
+          result-encoding: string
           script: |
             const workflow_run = await github.rest.actions.getWorkflowRun({
               owner: context.repo.owner,