Show keep button for locked artifacts

What does this MR do?

• Shows the keep button for builds with locked artifacts
• Improves the testing suite related to artifact locking

Related #229936 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• N/A Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• N/A Database guides
• N/A Separation of EE specific content