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
```

Start by reviewing the installation instructions in the README and the installer invoked through https://gh.io/copilot-install, then reproduce the VERSION="v1.0.75" command from the issue. Trace how the requested version is handled and verify that the installed binary reports v1.0.75 rather than the latest release.

Written by the indexing model from the issue text.