fix(ci): AUT-2165 serialize approval queue runs

[svcnemo-autobot]

Background

The new completion trigger can overlap scheduled or concurrent completion events.

What changed

Serialize approval-queue workflow runs without cancelling queued executions.

Details

flowchart LR
  A[Queue event] --> B[Serialized approval run]
  B --> C[Capacity check and approval]

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Tested

• Validated the concurrency group and non-cancelling policy.
• Confirmed the diff changes only the approval workflow.

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[svcnvidia-nemo-ci]

/ok to test f2dbc62