Add missing 'Keep this build forever' button to new build page

[subwaycookiecrunch]

Fixes #26196

The new build page (new-build-page.flag) was missing the "Keep this build forever" toggle because OverviewTab/index.jelly never included logKeep.jelly. The old build page had it in the controls section of index.jelly. Added the include to the details-bar.

Testing done

Ran a local Jenkins instance with new-build-page.flag enabled, created a freestyle job, ran a build, and confirmed the "Keep this build forever" button now shows up in the overview tab's details bar. Toggling it correctly switches to "Don't keep this build forever". Also confirmed the old build page still works fine with the flag disabled.

This is a single Jelly include — logKeep.jelly handles all the permission checks internally, so there's no new logic to unit test.

Screenshots (UI changes only)

Before

"Keep this build forever" button is absent from the new build page.

After

"Keep this build forever" button appears in the details bar next to the edit description button.

Proposed changelog entries

• Show the "Keep this build forever" toggle on the new build page

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
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• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@janfaracik

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

[janfaracik]

Thanks for contributing. Going to close this as a duplicate as this work has been done in #11204