Add Linux aarch64 support to install.sh

[cwoolum]

Problem

The Linear Release CLI ships a linear-release-linux-arm64 asset as of v0.9.0 (PR linear/linear-release#41), but this action's install.sh still hard-rejects Linux aarch64:

::error::Unsupported Linux arch: aarch64. Only x86_64 is supported.

This breaks the action on every ARM64 Linux runner — RunsOn pool=*-arm, GitHub-hosted ubuntu-*-arm, self-hosted Graviton, etc.

Fix

Map aarch64 / arm64 uname -m output to the new linear-release-linux-arm64 asset, mirroring the existing macOS arch-detection branch.

Test plan

• install.sh syntax check (bash -n install.sh)
• Verified the asset URL exists: https://github.com/linear/linear-release/releases/download/v0.9.0/linear-release-linux-arm64
• CI on a Linux arm64 runner downloads the correct asset

Downloading https://github.com/linear/linear-release/releases/download/v0.9.0/linear-release-linux-arm64
Run set -euo pipefail
Running: linear-release sync --release-version=a02dfb1dbb7deccd156185fd70bbc9474b43af05
=> Using custom release version: a02dfb1dbb7deccd156185fd70bbc9474b43af05
=> Found 1 commit between a02dfb1dbb7deccd156185fd70bbc9474b43af05 and a02dfb1dbb7deccd156185fd70bbc9474b43af05
=> No issue keys found
=> Issues [] and pull requests [161] have been added to release New release
=> Finished

[cwoolum]

@RomainCscn , is this something you would consider merging?