[Refactor] Use startsWith in platformAndArch

[gonzaloriestra]

WHY are these changes introduced?

Replaces regular expression matching platform.match(/^win.+/) with the standard platform.startsWith('win') method in platformAndArch.

WHAT is this pull request doing?

• Simplifies string inspection logic in platformAndArch (packages/cli-kit/src/public/node/os.ts).
• Preserves all existing observable behavior and type definitions.

How to test your changes?

CI

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PR created automatically by Jules for task 14961170502515935592 started by @gonzaloriestra

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