Add timeout for sign and publish step

[mrodm]

This PR adds a timeout step for the sign and publish step:

• https://buildkite.com/docs/pipelines/command-step#timeout_in_minutes

There is some condition or scenario where the Jenkins build is not triggered and the script waits for that job forever.
In order to reduce the waiting time, a timeout of 30 minutes is set for that step. Usually that step is completed in 10 minutes or so.

The output in those builds are:

2023/03/22 10:18:22 Triggering job: elastic+unified-release+master+sign-artifacts-with-gpg
2023/03/22 10:18:22 job.go:428: elastic+unified-release+master+sign-artifacts-with-gpg is already running

Relates #1200

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 77485cc

cc @mrodm