fix(ci): simplify Bun setup for Windows ARM64

[oddrationale]

Before Submitting This PR

Please confirm you have done the following:

• I have searched existing issues and pull requests (including closed ones) to ensure this isn't a duplicate
• I have read CONTRIBUTING.md

If this is a feature or change that was previously closed/rejected:

• I have explained in the description below why this should be reconsidered
• I have gathered community feedback (link to discussion below)

Human Written Description

Bun now releases aarch64 native builds for Windows. This removes the previous work-around I implemented when originally creating the aaarch64/arm64 Windows build. We can now have a single, unified bun setup step.

Related Issues/Discussions

Discussion:

• Original aarch64 implementation: Enable Windows ARM64 builds in CI #340
• Upstream change to the setup-bun action: fix: use native Windows ARM64 binary for Bun >= 1.3.10 oven-sh/setup-bun#165 (merged)
• Follow-up report/closure: Downloading x64 version on arm64 Windows runners oven-sh/setup-bun#168 (closed)

Community Feedback

N/A (CI/workflow cleanup based on upstream action behavior)

Testing

• Confirmed workflow runs and selects the the aarch64 version on Windows. https://github.com/oddrationale/Handy/actions/runs/22690953409/job/65786029286#step:5:6
• Note: The build fails because I don't have the code signing certificates. But the Setup Bun step selects the correct bun architecture and build correctly.

Screenshots/Videos (if applicable)

N/A

AI Assistance

• No AI was used in this PR
• AI was used (please describe below)

If AI was used:

• Tools used: Codex (gpt-5.3-codex) and Claude Code (Opus 4.6)
• How extensively: Perform code review and draft PR.