fix: Enable x64 and arm64 cross-platform builds and fix installer CPU arch detection

[postEntropy]

Description

Users with x86_64 machines were encountering an Exec format error when trying to install and run the application using installer.sh.

The Problem

• The GitHub Actions workflow (ubuntu-latest) implicitly runs on an arm64 runner in some setups (or vice-versa), causing electron-builder to produce only arm64 binaries.
• The installer.sh hard-coded the AppImage URL to a generic linear-linux-${VERSION}.AppImage, downloading the ARM binary regardless of the host machine's architecture.

The Solution

1. GitHub Actions Workflow (release.yml)
   • Explicitly separate builds for --x64 and --arm64.
   • Store the artifacts in an intermediate dist-final directory before releasing, avoiding electron-builder's default behavior, which cleans the dist folder on each consecutive build command.
2. Installation Script (installer.sh)
   • Added ARCH=$(uname -m) to detect the host architecture upon installation.
   • Using x86_64 or aarch64, the script now successfully routes download requests to Linear-${VERSION}.AppImage or Linear-${VERSION}-arm64.AppImage.
   • Also addressed a minor output error by removing the redundant and conflicting chown/chmod of /opt/Linear/chrome-sandbox at the end of the script, since the sandbox root is already perfectly handled a few lines earlier inside ${INSTALL_DIR}/squashfs-root/chrome-sandbox.

Testing instructions

I have built the artifacts locally simulating Linux x64 using npx electron-builder --linux AppImage deb --x64.
Running APPIMAGE_URL="file:///path/to/local/Linear-0.2.4.AppImage" ./installer.sh installs flawlessly, creates the Desktop entry, and opens the application successfully on x86_64.