Fix binary build time returned by version command

[swiatekm]

What does this PR do?

Fixes a bug where the build time reported by the agent binary is the zero value. This was accidentally broken by #12856, where the global containing this value was moved to the Settings struct, but was never actually populated.

I've also added unit tests and verified that the output of version is correct in an existing integration test.

Why is it important?

The reported build time should be accurate.

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

How to test this PR locally

Build the agent and run elastic-agent version --binary-only. Packaging isn't necessary.

[mergify]

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

• backport-./d./d is the label that automatically backports 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.

[elasticmachine]

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

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: ff7e525

Failed CI Steps

• Win2025:non-sudo:default

History

• 💔 Build #35756 failed e37a226

cc @swiatekm

[github-actions]

@Mergifyio backport 8.19 9.2 9.3

[mergify]

backport 8.19 9.2 9.3

✅ Backports have been created

Details

• #13153 [8.19] (backport #13140) Fix binary build time returned by version command has been created for branch 8.19
• #13154 [9.2] (backport #13140) Fix binary build time returned by version command has been created for branch 9.2
• #13155 [9.3] (backport #13140) Fix binary build time returned by version command has been created for branch 9.3