[8.x](backport #5658) Fix docker image build when multiple platforms are specified

[mergify]

What does this PR do?

This PR extends the behaviour introduced in #5647 by copying platform-independent components into /archive/ instead of moving them.

This fixes the build of docker image service when multiple platforms are specified for the build: without this fix the platform-independent component would be moved exactly once, in the directory of the first platform considered during the build.

Note:
It seems that EXTERNAL=true SNAPSHOT=true mage package still suffers from some inconsistencies when downloading connectors dependency for multiple platforms but this is unrelated to this fix and will be tracked by a separate issue.

Why is it important?

To correctly build elastic-agent-service docker image for multiple platforms during the Unified Release.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

---

This is an automatic backport of pull request #5658 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b8a685c has failed:

On branch mergify/bp/8.x/pr-5658
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit b8a685c29c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   dev-tools/mage/dockerbuilder.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   magefile.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[pchila]

This is needed only if the other backport PR #5634 is merged. If that's not the case we can simply close it

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[pchila]

Putting this in draft for the moment until we come to a decision about #5634

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pchila? 🙏