CI: resolve container Python for release builds

[gyohuangxin]

Summary

• Resolve the Python executable inside non-manylinux release build containers before installing dependencies
• Use the resolved Python with -m pip for legacy dependency installation steps
• Build legacy wheels with the same resolved Python, allowing custom containers such as rocm/ufb-private that do not expose pip/python3 like rocm/pytorch

Testing

• python3 YAML parse of .github/workflows/aiter-release.yaml
• git diff --check

[github-actions]

🏷️ CI Guide

Runs automatically on every PR:

• ✅ Pre-checks (submodule verification, code formatting)
• ✅ Aiter op tests (gfx942 + gfx950)
• ✅ Triton tests on MI35X (only when aiter/ops/triton/** or related paths are changed)

Extended tests (opt-in via labels):

Label	Tests
ci:triton-300x	Run an additional Triton test job on MI300X in PRs; main branch always runs both MI35X and MI300X
ci:sglang	SGLang integration tests: DeepSeek-R1-MXFP4 accuracy, Qwen 3.5 accuracy
ci:atom	ATOM benchmark: DeepSeek-R1-0528, GPT-OSS-120B
ci:atom_full	ATOM accuracy suite for PR and main models from ATOM models_accuracy.json
ci:vllm	vLLM benchmark: GPT-OSS-120B, DeepSeek-R1-0528, Kimi-K2.5
ci:all	All standard extended tests (excludes ci:atom_full)

Only add ci:atom_full for FlyDSL or Triton upgrades.
Add labels via the sidebar or gh pr edit 4624 --add-label <label>

[Copilot]

Pull request overview

This PR updates the release GitHub Actions workflow to locate a usable Python executable inside “legacy” (non-manylinux) build containers and then consistently use that interpreter for pip installs and wheel builds, improving compatibility with custom containers that don’t expose pip/python3 on PATH.

Changes:

• Add a “Resolve container Python (legacy)” step that finds a Python interpreter with pip available and exports it as CONTAINER_PYTHON.
• Switch legacy dependency installation steps from pip ... to "${CONTAINER_PYTHON}" -m pip ....
• Build legacy wheels using the resolved Python rather than hardcoding python3.

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