Installing a Specific Version Always Installs the Latest Version

### Describe the bug

I wanted to downgrade Copilot in Docker Sandbox because the latest version is not working correctly. Following the installation instructions from the README, I attempted to install version v1.0.75. However, regardless of the version specified, the installer always installs the latest available version.

Reproduced in WSL and Docker Sandboxes

### Affected version

_No response_

### Steps to reproduce the behavior

1. Install latest version `curl -fsSL https://gh.io/copilot-install | bash`
2. Delete installed binary `rm ~/.local/bin/copilot`
3. Attempt to install a specific version `curl -fsSL https://gh.io/copilot-install | VERSION="v1.0.75" bash`

### Expected behavior

Version v1.0.75 is installed

### Additional context

```
agent@copilot:/$ curl -fsSL https://gh.io/copilot-install | VERSION="v1.0.75" bash
Installing GitHub Copilot CLI...
Downloading from: https://github.com/github/copilot-cli/releases/download/v1.0.75/copilot-linux-x64.tar.gz
✓ Checksum validated
✓ GitHub Copilot CLI installed to /home/agent/.local/bin/copilot

Installation complete! Run 'copilot help' to get started.
agent@copilot:/$ copilot --version
GitHub Copilot CLI 1.0.77.
Run 'copilot update' to check for updates.
agent@copilot:/$ which copilot
/home/agent/.local/bin/copilot
```

先檢視 README 中的安裝說明，以及透過 https://gh.io/copilot-install 呼叫的安裝程式，接著重現 issue 中的 VERSION="v1.0.75" 命令。追蹤要求的版本如何被處理，並確認已安裝的二進位檔回報的是 v1.0.75，而不是最新版本。

由索引模型根據 Issue 內容生成。