fixes #21106

When a run was caused by an upstream change that itself was triggered by an SCMTriggerCause, the link on the run to the pollingLog of the upstream build was lost after a Jenkins restart.
The root cause of the problem is the fix for #23334 which removed the onLoad(Run build) from the upstream cause which effectively prevents loading of upstream causes after a restart. The onLoad(Job, job, int buildNumber) was then actually never called.
The fix should not reintroduce the issue from #23334, as the DeeplyNestedUpstreamCause will at one point jump in and prevent further loading of more builds.

Testing done

Added a test that fails without the fix and succeeds afterwards.
Also verified manually:

1. Create a job downstream that does nothing
2. Create a job upstream, that uses Git SCM with any small github project
3. add a post build step that triggers a build of downstream
4. configured Poll SCM to run every minute
5. wait for the upstream and downstream jobs to have run
6. The downstream build now has a working link in it's SCMTrigger cause
7. Restart Jenkins and visit the downstream build
8. The downstream build still has a working link in it's SCMTrigger cause

Before the fix step 8 returned a broken link.

Screenshots (UI changes only)

Before

After

Proposed changelog entries

• Fix link to polling log of upstream build caused by an SCM trigger after a Jenkins restart

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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