[8.14] [CI] Fix missing test failure annotations (#186446) (#186474)

# Backport

This will backport the following commits from `main` to `8.14`:
- [[CI] Fix missing test failure annotations
(#186446)](#186446)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Alex
Szabo","email":"[email protected]"},"sourceCommit":{"committedDate":"2024-06-19T14:55:18Z","message":"[CI]
Fix missing test failure annotations (#186446)\n\n## Summary\r\nrename
`SLACK_NOTIFICATIONS_ENABLED`
=>\r\n`ELASTIC_SLACK_NOTIFICATIONS_ENABLED` to follow up on
elastic-wide\r\nbuildkite changes.\r\nThis should re-enable test failure
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Fix missing test failure
annotations","number":186446,"url":"https://github.com/elastic/kibana/pull/186446","mergeCommit":{"message":"[CI]
Fix missing test failure annotations (#186446)\n\n## Summary\r\nrename
`SLACK_NOTIFICATIONS_ENABLED`
=>\r\n`ELASTIC_SLACK_NOTIFICATIONS_ENABLED` to follow up on
elastic-wide\r\nbuildkite changes.\r\nThis should re-enable test failure
listing on the slack errors we
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Fix missing test failure annotations (#186446)\n\n## Summary\r\nrename
`SLACK_NOTIFICATIONS_ENABLED`
=>\r\n`ELASTIC_SLACK_NOTIFICATIONS_ENABLED` to follow up on
elastic-wide\r\nbuildkite changes.\r\nThis should re-enable test failure
listing on the slack errors we
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Co-authored-by: Alex Szabo <[email protected]>

.buildkite/pipeline-utils/test-failures/annotate.ts

@@ -177,7 +177,7 @@ export const annotateTestFailures = async () => {
     );
   }
 
-  if (process.env.SLACK_NOTIFICATIONS_ENABLED === 'true') {
+  if (process.env.ELASTIC_SLACK_NOTIFICATIONS_ENABLED === 'true') {
     buildkite.setMetadata(
       'slack:test_failures:body',
       getSlackMessage(failures, failureHtmlArtifacts)