Buildkite needs a 'mage package' step separate from integration tests

[cmacknz]

Right now Buildkite does not run mage package for all platforms, the package target is only executed for the platforms covered by integration tests.

• Add a mage package build step that mimics what the unified snapshot release job does to ensure new changes do not break the build.
• The .tar.gz and .zip artifacts for each PR should be available as Buildkite artifacts. This would simplify manual testing of PRs by allowing the changes to be downloaded and installed easily.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)