What does this PR do?

Revert changes introduced with #7805 (there's a side effect as the filenames for fips artifacts are changed to elastic-agent-fips-core due to the usage of qualifier.
This breaks also elastic-agent-core uploads in elastic-agent-binary-dra pipeline: https://buildkite.com/elastic/elastic-agent-binary-dra/builds/3060
Fix rootDir of the elastic-agent-core artifacts in packages.yaml

Why is it important?

To have consistent naming of artifacts

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

How to test this PR locally

Related issues

• Supersedes Modify fips core spec qualifier and name #7818

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?
• ...