[CI] Fix project build & deploy job

[delanni]

Summary

In #195581 we've added the option to deploy through the clickable triggers. But in it's current state, it's broken in several aspects.

(1) It's not starting on click. Triggers was resulting in a 422 on Buildkite's side, and after digging more into it, this was the error: Apparently, building PRs needs to be enabled on jobs that want to be triggered through the PR bot.

(2) It is set up to run regardless of the labels

(3) There's no feedback on runs

Changes

This PR:

• enables buildability in the pipeline's config
• exits early if deploy labels are missing
• adds a comment on the PR if a deploy job is started or finished
• removes the kibana build step, it's not needed, as we have a step to build the docker image

TODO:

• [x] Add feedback about a started job (either through a non-required check, or a github comment)
• [x] Early exit if a label is missing

There are several other builds started right now, because the logic that would trigger a build on changing a draft to ready. To be fixed in https://github.com/elastic/buildkite-pr-bot/issues/78

Tested after manually by enabling the option on the UI, and triggering through the checkbox: https://buildkite.com/elastic/kibana-deploy-project-from-pr/builds/23