Inject `PNPM_CONFIG_RUNTIME=false` when `vp env` is `managed`

### Description

`pnpm` itself has a runtime version management system, means the same Node.js version might be downloaded twice by both Vite+ and pnpm.

### Suggested solution

When `vp env` is set to `managed` mode, we can inject `PNPM_CONFIG_RUNTIME=false` in the shell script.

Also, when users call `vp env off` / `vp env on`, Vite+ should also change the status of this env variable.

This can be also shown in `vp env doctor` (Suggested by @ TheAlexLichter)

### Alternative

N / A

### Additional context

Context: https://discord.com/channels/1475973262193459293/1479410337223802951/1538525485166760026

### Validations

- [x] Read the [Contributing Guidelines](https://github.com/voidzero-dev/vite-plus/blob/main/CONTRIBUTING.md).
- [x] Confirm this request is for Vite+ itself and not for Vite, Vitest, tsdown, Rolldown, or Oxc.
- [x] Check that there isn't already an issue requesting the same feature.