ci: Fix event name reference in CI workflow condition for merge group

[balasaajay]

What does this PR do ?

Fix event name reference in CI workflow for merge group condition

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[balasaajay]

/ok to test

[copy-pr-bot]

/ok to test

@balasaajay, there was an error processing your request: E1

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[balasaajay]

/ok to test 70332f9

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25043536392