fix: export corepack env vars + disable vp env node management

[vltbaudbot]

Problem

Since the vp PR (#76) was merged, the run/run benchmark job fails consistently on every CI run:

Benchmark 5: pnpm
Error: The preparation command terminated with a non-zero exit code.

Failing runs: 23092848951, 23108493842, 23139487059

Root Causes (two issues)

1. vp installer hijacks Node.js

The vp installer auto-enables Node.js version management when $CI is set (always true in GitHub Actions). This creates shims at ~/.vite-plus/bin/node, npm, npx that intercept all Node.js calls.

Evidence: npm stayed at 11.9.0 instead of upgrading to 11.11.1 after npm install -g npm@latest.

2. Corepack env vars not persisted across CI steps

setup.sh sets COREPACK_ENABLE_STRICT=0 and COREPACK_ENABLE_AUTO_PIN=0, but these were never exported or written to $GITHUB_ENV. They only applied within the setup step itself.

In the benchmark step, corepack 0.34+ runs with defaults:

• COREPACK_ENABLE_STRICT=1: Refuses to run a PM that doesn't match the packageManager field. Since zpm's prepare sets packageManager=yarn@6.0.0-rc.15, and the cleanup may not fully restore it (the git checkout -- packages/ package.json in clean_workspace_protocol fails silently when packages/ doesn't exist in the run fixture), pnpm's prepare sees packageManager=yarn and corepack immediately rejects it (exit code 1 in ~0.04s).
• COREPACK_ENABLE_AUTO_PIN=1: Corepack auto-adds a packageManager field when one doesn't exist, contaminating the fixture between iterations.

Reproduction (corepack 0.34.6):

echo '{"name":"test","packageManager":"yarn@6.0.0-rc.15"}' > package.json
COREPACK_ENABLE_STRICT=1 corepack pnpm@latest install --silent
# → "This project is configured to use yarn" (exit 1)
COREPACK_ENABLE_STRICT=0 corepack pnpm@latest install --silent
# → works fine (exit 0)

Fix

1. vp env off after installing vp to disable its Node.js shims
2. Export + persist COREPACK_ENABLE_STRICT=0 and COREPACK_ENABLE_AUTO_PIN=0 to $GITHUB_ENV so all CI steps inherit them