fix: improve browserslist cache key

[chenjiahan]

Summary

• Updated the browserslist cache key to separate path and environment values, preventing collisions between different project paths or NODE_ENV combinations.

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Codex Task

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[Copilot]

Pull Request Overview

This PR improves the browserslist cache key generation by replacing string concatenation with a template literal that uses a colon delimiter. The associated regression test was removed from helpers.test.ts as mentioned in the PR description.

• Improved cache key format to use ${path}:${env ?? ''} instead of path + env
• Removed browserslist cache regression test

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