Test `buildkite_pipeline_upload` action in CI

[mokagio]

What does it do?

Calls the new buildkite_pipeline_upload action in CI to upload a dedicated pipeline and have an integration-level test for the action.

See https://github.com/wordpress-mobile/release-toolkit/pull/597#pullrequestreview-2333686032

Checklist before requesting a review

• [x] Run bundle exec rubocop to test for code style violations and recommendations
• [x] Add Unit Tests (aka specs/*_spec.rb) if applicable
• [x] Run bundle exec rspec to run the whole test suite and ensure all your tests pass
• [x] Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section. – N.A.
• [x] If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider. – N.A.

[dangermattic]

	2 Warnings
:warning:	Please add an entry in the CHANGELOG.md file to describe the changes made by this PR
:warning:	PR is not assigned to a milestone.

Generated by :no_entry_sign: Danger

[iangmaia]

Thanks for this @mokagio! Looks like the build with the pipeline upload worked fine 🎉 I've merged #597 and released 12.2.0 for release-toolkit.

[AliSoftware]

@mokagio I just stumbled upon this old PR while looking at the opened PRs for this repo.

It this still relevant? If so, could benefit fixing conflicts on the pipeline file as well as some freshing-up (account for some new convention decisions made since like wrt to using the emoji in labels after all, adding git_commit_status notify blocks, updating the IMAGE_ID to a more recent one, …) 🙂

[mokagio]

@AliSoftware I'd say it's still relevant yet. It doesn't hurt to have these tests does it?

However, just to set expectations, I don't think I'll have time to address this before my support rotation next week subsequent AFK.