feat: support Linux arm64 architecture

[zimeg]

Changelog

Add Linux arm64 (aarch64) architecture support for installation and auto-update. Closes #276.

Summary

This pull request adds Linux arm64 support to the Slack CLI build, install, and auto-update pipelines.

• Goreleaser now builds both amd64 and arm64 binaries for Linux
• Install scripts (install.sh, install-dev.sh) detect architecture via uname -m for versions >= 4.1.0
• Auto-update (getUpdateFileName) returns architecture-specific Linux filenames
• Archive scripts produce linux_amd64, linux_arm64, and linux_64-bit (amd64 fallback) tarballs
• CI S3 upload patterns updated to catch all Linux archives
• Unknown architectures fall back to the linux_64-bit (amd64) binary

Preview

N/A — no UI changes.

Testing

• On a Linux amd64 machine, run bash scripts/install.sh and verify it downloads the linux_amd64 tarball
• On a Linux arm64 machine (e.g. Raspberry Pi, Asahi Linux), run bash scripts/install.sh and verify it downloads the linux_arm64 tarball
• Install an older version (bash scripts/install.sh -v 4.0.1) and verify it falls back to linux_64-bit

Notes

• Version threshold is 4.1.0 — older versions only had linux_64-bit archives, so the install scripts fall back for those
• The linux_64-bit archive is always a copy of the amd64 build, preserving backwards compatibility for users on older install scripts or unknown architectures
• Mirrors the existing macOS approach (amd64, arm64, 64-bit universal)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.29%. Comparing base (bd6a088) to head (6ba84a4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #539      +/-   ##
==========================================
- Coverage   71.30%   71.29%   -0.01%     
==========================================
  Files         222      222              
  Lines       18706    18712       +6     
==========================================
+ Hits        13338    13341       +3     
- Misses       4188     4189       +1     
- Partials     1180     1182       +2     

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