fix(install): select baseline Bun builds without AVX2

[liangmiQwQ]

Close #2041

Bun's standard x64 builds require AVX2. This PR detects AVX2 before a fresh managed Bun download and selects Bun's baseline package on older x64 CPUs.

This is an intentionally scoped temporary fix. Existing cached Bun installations keep using their current files and are not replaced or migrated.

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[liangmiQwQ]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: c46b34c00f

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: e90995c236

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

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[liangmiQwQ]

The same problem as #2151, the fix cannot influence the already installed Buns. I don't add a legacy refresh logic in this PR, Maybe just let users run our new vp env clean?