ci: fix Vulkan GPU gate runner path initialization

[Irk2wd]

Summary

• move GPU build/output path initialization out of job-level env, where the runner context is unavailable
• derive the same per-run paths from RUNNER_TEMP inside a Windows PowerShell step
• preserve evidence paths, trust boundaries, and all downstream environment-variable contracts

Root cause

The first post-merge dispatch of Phase 27 was rejected by GitHub before creating a run: runner.temp is not an allowed expression context in jobs.gpu_validation.env.

Validation

• GitHub service accepted branch dispatch run https://github.com/NJUCG/MoerEngine/actions/runs/30695539710 for exact head e78934c51ab718667c0dcf17ea0b80db84e4301a
• branch dispatch then failed closed as designed with manual_dispatch_requires_main; GPU job was skipped
• Python runner contracts: 164/164 PASS
• PowerShell 5.1 AST and spaced-path probe: PASS
• YAML parse and git diff --check: PASS
• independent P0-P2 review: clean

After merge, the workflow will be dispatched on main for the real protected-environment GPU qualification.

[Irk2wd]

@codex review

Please review exact head e78934c51ab718667c0dcf17ea0b80db84e4301a, focusing on whether moving runner.temp from job-level env to a PowerShell step using RUNNER_TEMP/GITHUB_ENV is valid on Windows PowerShell 5.1, whether all downstream steps receive BUILD_DIR/OUTPUT_DIR, and whether this creates any fail-open or injection issue. Report remaining P0-P2 issues.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: e78934c51a

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