ci: Update workflow to use same commit for building docker image and running tests

[balasaajay]

What does this PR do ?

Update workflow by introducing a SHA resolution step to ensure consistent commit references across jobs. This replaces the previous method of obtaining the merge commit SHA and updates downstream jobs to use the resolved SHA from the configure step.

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[balasaajay]

/ok to test e7c6ffe

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25830877082

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25831006107