ci: install alignment environments only once

[zrr1999]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do?

The workflow calls setup_venvs.sh before run_alignment_test.sh, which already calls it. This installs the environment twice and starts a second attempt after the first setup fails. Remove the redundant workflow call so the test entrypoint owns setup; its existing set -euo pipefail and the workflow exit-status check propagate installation failures.

Validation: after a normal merge of current PFCCLab main, the PR diff remains one removed workflow command. Repository pre-commit checks and tools/autoformat.sh in check mode passed. The actual workflow shell tail and test entrypoint passed four isolated command replays: success, setup failure, model failure, and comparison failure. Setup runs exactly once; its failure starts no models, and all failure paths return nonzero. The unchanged upstream baseline reproduces duplicate setup. Real GPU alignment passed in the current CI run: MinimaxV2.5 EP2, two ranks and ten training steps, with identical per-token and final-loss MD5 values for every rank and step.

This removes duplicate setup invocations; it does not change the package manager's own download retries or fix network connection resets.

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Related failure: https://github.com/PFCCLab/Megatron-LM/actions/runs/34825280415/job/103915866438

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