Update docs-preview link (elastic#176468)

## Summary

Following the migration from Jenkins to Buildkite, docs previews are now
available at <repo>_bk_<PR>.
More context in elastic/docs#2898

### Checklist

### Risk Matrix

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

(cherry picked from commit 096bf55)

Author: nkammah

.buildkite/scripts/lifecycle/post_build.sh

@@ -12,7 +12,7 @@ fi
 node "$(dirname "${0}")/ci_stats_complete.js"
 
 if [[ "${GITHUB_PR_NUMBER:-}" ]]; then
-  DOCS_CHANGES_URL="https://kibana_$GITHUB_PR_NUMBER}.docs-preview.app.elstc.co/diff"
+  DOCS_CHANGES_URL="https://kibana_bk_$GITHUB_PR_NUMBER}.docs-preview.app.elstc.co/diff"
   DOCS_CHANGES=$(curl --connect-timeout 10 -m 10 -sf "$DOCS_CHANGES_URL" || echo '')
 
   if [[ "$DOCS_CHANGES" && "$DOCS_CHANGES" != "There aren't any differences!" ]]; then