Propagate service account to affinity assistant

[wilderbridge]

Fixes #3748 and picks up #3751

Changes

• Propagate the PipelineRun taskRunTemplate.serviceAccountName to the affinity-assistant StatefulSet so the assistant pod uses the same service account as the TaskRuns.
• Added a unit test to verify the service account is copied onto the assistant pod template.

Submitter Checklist

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
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Release Notes

Fix affinity assistant pods to use the PipelineRun’s taskRunTemplate.serviceAccountName so they align with TaskRuns. 

[tekton-robot]

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[tekton-robot]

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[wilderbridge]

Really tried to make @tekton-robot happy about the release note but to no avail.