feat: add Ubuntu/Debian apt install support

[Khanmdaadil]

Summary

This PR adds support for installing GitHub Copilot CLI via apt package manager on Ubuntu/Debian systems.

Changes

• install.sh: Added apt package manager detection and installation support for Debian/Ubuntu systems

  • Auto-detects Debian-based systems (checks for /etc/debian_version)
  • Prompts user to use apt when running interactively
  • Added USE_APT=true environment variable for non-interactive installation
  • Sets up GitHub official apt repository with GPG key signing

• README.md: Added documentation for apt installation

  • Quick installation command using the install script
  • Manual installation steps for advanced users

Usage

Automatic (via install script)

curl -fsSL https://gh.io/copilot-install | USE_APT=true sudo bash

Manual Installation

# Add GitHub GPG key
curl -fsSL https://cli.github.com/packages/githubcli-archive-keyring.gpg | sudo gpg --dearmor -o /usr/share/keyrings/github-copilot-archive-keyring.gpg

# Add the repository
echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/github-copilot-archive-keyring.gpg] https://cli.github.com/packages stable main" | sudo tee /etc/apt/sources.list.d/github-copilot.list > /dev/null

# Update and install
sudo apt update && sudo apt install -y copilot-cli

Testing

• Tested on Ubuntu 22.04 LTS
• Verified both interactive and non-interactive installation modes