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What does this PR do?

Update BuildData creation logic: when a new instance is created, a check is made to see whether the associated run already refers to another instance that was created and updated during previous pipeline stages.
If there is a previously created instance, it is used to populate Git values in the new instance, in case they are missing.

For instance, if there are environment variables that are set inside a pipeline stage (e.g. Git branch and repo URL that are set during SCM checkout), these variables will only be available during that stage: later stages will not be able to use them.
The fallback mechanism ensures that these variables are propagated whenever a new BuildData instance is created at some later point.

This fixes https://datadoghq.atlassian.net/browse/AGENT-10162

Description of the Change

Alternate Designs

Possible Drawbacks

Verification Process

Verified manually using a dockerized Jenkins instance.

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
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