Add voice mode support for linux-arm64

### Describe the feature or problem you'd like to solve

Voice mode cannot be installed when running the GitHub Copilot CLI on **linux-arm64**. The voice runtime install fails with:

```
Voice runtime install failed: Error: Voice mode is not supported on linux-arm64. Supported platforms: win32-x64, win32-arm64, linux-x64, darwin-arm64.
```

Notably, both `win32-arm64` and `darwin-arm64` **are** supported, so ARM64 itself is not the blocker — only the **Linux + ARM64** combination is missing. This leaves Linux ARM64 users (e.g. Ampere/Graviton cloud dev boxes, ARM64 laptops/VMs, Raspberry Pi-class devices) unable to use voice mode at all.

### Proposed solution

Ship a `linux-arm64` build/prebuilt of the voice runtime so voice mode is available on Linux ARM64, bringing it to parity with the other ARM64 platforms that are already supported. If a native dependency is the constraint, providing a prebuilt binary or a documented build path for linux-arm64 would unblock these users.

### Example prompts or workflows

- On a Linux ARM64 cloud dev box, run `copilot` and use voice input to dictate a prompt hands-free.
- Use voice mode on an ARM64 Linux laptop/VM the same way it already works on `darwin-arm64` and `win32-arm64`.
- Voice-driven workflows in CI/dev containers running on ARM64 Linux hosts (Graviton/Ampere).

### Additional context

- Platform: `linux-arm64`
- Reported supported platforms: `win32-x64`, `win32-arm64`, `linux-x64`, `darwin-arm64`
- `darwin-arm64` and `win32-arm64` are supported, so the gap is specifically the Linux ARM64 target.

Error as shown in the CLI:

```
X Voice runtime install failed: Error: Voice mode is not supported on linux-arm64. Supported platforms: win32-x64, win32-arm64, linux-x64, darwin-arm64.
```

Start by locating the voice runtime installer and the platform support list that emits the reported error. Check how the existing linux-x64, win32-arm64, and darwin-arm64 runtimes are packaged, then determine whether a linux-arm64 prebuilt or documented build path is feasible. Done means voice mode installs and runs on linux-arm64.

索引モデルが issue の本文から書いたものです。