fix: push the cloud images from packaging steps

[pkoutsovasilis]

What does this PR do?

During the backport of #9282 I missed the fact that this #8337 never got backported to 9.1. This leads to certain failures context here

So, this PR ensures that cloud images built as part of the packaging steps for integration tests are properly pushed to the Docker registry. Previously, this push step was part of a separate FIPS-specific step (integration-fips-cloud-image), which has now been removed.

The logic to load and push the cloud image has been relocated to the packaging steps themselves (packaging-containers-x86-64-fips and others), specifically using the new integration-cloud-image-push.sh script. In addition, a shared Vault Docker login plugin has been introduced and reused via anchors across these steps.

Why is it important?

This fix ensures that the required images for FIPS integration tests are available without relying on a separate Buildkite step and most importantly they do use the USE_PACKAGE_VERSION env var

Checklist

• I have read and understood the pull request guidelines of this project.
• 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

None. This is a CI-only change and has no impact on users of the Elastic Agent.

How to test this PR locally

This is a CI-only change thus maybe open a draft PR

Related issues

• N/A

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 89c57b2

cc @pkoutsovasilis

[elasticmachine]

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

[dliappis]

LGTM