Add aarch64-windows-gnu build target

[robobun]

Summary

• Add arm64 Windows (aarch64-windows-gnu) builds to the CI matrix
• Includes both ReleaseFast and ReleaseSafe variants
• The gnu ABI is the correct one for aarch64-windows per zig's target definitions in lib/std/zig/target.zig

Test plan

• CI builds successfully for the new aarch64-windows-gnu target

🤖 Generated with Claude Code

[coderabbitai]

Caution

Review failed

The pull request is closed.

Walkthrough

Adds aarch64-windows-gnu as a new build target to the GitHub Actions workflow matrix, including build commands, executable permission configuration for zig.exe, ZIP artifact creation, and release artifact extensions for the new target variants.

Changes

Cohort / File(s)	Summary
GitHub Actions Workflow Configuration \.github/workflows/bun_build.yaml``	Introduces aarch64-windows-gnu build target matrix entry; adds executable permission steps for zig.exe in aarch64-windows-gnu variants; adds build commands for new target; creates ZIP artifacts for bootstrap-aarch64-windows-gnu and bootstrap-aarch64-windows-gnu-ReleaseSafe; extends release artifact list with new bootstrap ZIP files

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_{Comment @coderabbitai help to get the list of available commands and usage tips.}