Revert "bk: run faster builds"

[v1v]

Reverts #7029

As long as the post-checkout does some logic this cannot be used.

In theory, the BK PR bot could be the solution, and could help to remove all the post-checkout logics required to always test PRs with the merge, but I dont' think it's available yet that feature

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[ebeahan]

I also reverted #7037, since its purpose was to try to fix the changes from #7029.

[ebeahan]

Actually, we may not need to revert. Synthetics are now running again: https://buildkite.com/elastic/gpctl-promote-after-serverless-devenv-synthetics/builds/43741

Let me validate before we merge.

[v1v]

I think we need to rebump this.

The current checkout process with the post-checkout step is not working as expected:

• https://buildkite.com/elastic/fleet-server/builds/14781#019e4149-3bfa-41ce-a41e-1b17fb5a1d4e

HEAD is now at ab3b3a1 chore: Update snapshot.yml (#7060)
--
  | Current branch: HEAD
  | Switched to a new branch 'pr_merge_7068'
  | New branch created: pr_merge_7068
  | fatal: refusing to merge unrelated histories
  | Merge failed: 128
  | fatal: There is no merge to abort (MERGE_HEAD missing).

I'm not much familiar with the post-checkout script, but it's causing some issues when using the spare-checkout.

One solution could be to add a condition in the post-checkout so it does not run when running the upload command