Allow installing prerelease CLI with install.sh

[ellismg]

Does what it says on the tin.

[Copilot]

Pull request overview

This PR adds support for installing prerelease versions of the GitHub Copilot CLI via the install.sh script. Users can now set VERSION="prerelease" to install the latest prerelease version.

Changes:

• Restructured version handling logic to support three cases: latest/empty, prerelease, and specific version
• Added prerelease detection using git ls-remote to fetch the latest tag
• Moved the default (latest) case to the first condition for clarity

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[ellismg]

Confirmed this works:

matell@Matts-Work-MacBook copilot-agent-runtime % curl -fsSL https://raw.githubusercontent.com/github/copilot-cli/refs/heads/ellismg/support-prerelease-for-install-sh/install.sh | VERSION=prerelease bash
Installing GitHub Copilot CLI...
Latest prerelease version: v0.0.389-1
Downloading from: https://github.com/github/copilot-cli/releases/download/v0.0.389-1/copilot-darwin-arm64.tar.gz
✓ Checksum validated
Notice: Replacing copilot binary found at /Users/matell/.local/bin/copilot.
✓ GitHub Copilot CLI installed to /Users/matell/.local/bin/copilot

Installation complete! Run 'copilot help' to get started.
matell@Matts-Work-MacBook copilot-agent-runtime %

Also validated that without the VERSION=prerelease, we still install latest as before.