[Flaky Test]: Trigger Serverless Beats Tests – `curl: (22) The requested URL returned error: 503`

### Failing test case

Trigger Serverless Beats Tests

### Error message

curl: (22) The requested URL returned error: 503

### Build

https://buildkite.com/elastic/beats/builds/36953/steps/canvas?sid=019ab6a5-5360-4f06-a197-bec81bc909ff#019ab6a5-5404-46e8-8339-0871569bd816/178-181

### OS

Linux, Mac, Windows

### Stacktrace and notes

```markdown
$ /buildkite-agent/plugins/bk-agent-prod-k8s-1764000901829543508/github-com-buildkite-plugins-monorepo-diff-buildkite-plugin-v1-0-1/hooks/command
curl: (22) The requested URL returned error: 503
failed to download https://github.com/buildkite-plugins/monorepo-diff-buildkite-plugin/releases/download/v1.0.1/monorepo-diff-buildkite-plugin_Linux_amd64
🚨 Error: The command exited with status 1
user command error: running "plugin monorepo-diff-buildkite-plugin command" shell hook: The plugin monorepo-diff-buildkite-plugin command hook exited with status 1
```

Start with the linked Buildkite build and the “Trigger Serverless Beats Tests” failure. Inspect the shell hook command and the monorepo-diff-buildkite-plugin release download that returns HTTP 503. Done means the trigger test completes without the plugin download failing with a 503.

Written by the indexing model from the issue text.