fix(ci): set CUDA major for torch-memory-saver lock generation

[balasaajay]

What does this PR do ?

torch-memory-saver requires TMS_CUDA_MAJOR when uv invokes its build
backend during lockfile generation. Derive it from the configured CUDA
base image to avoid hardcoding and keep it aligned with the container.

fixes: https://github.com/NVIDIA-NeMo/RL/actions/runs/33016571935/job/98337729453

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[kajalj22]

/ok to test 2e555db

[balasaajay]

/ok to test f08a3f2