Suggest pnpm in missing modules error

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### What were you expecting to happen?

If node_modules aren't present and `pnpm-lock.yaml` is present, gulp-cli should say "Try running: pnpm install"

### What actually happened?

It defaults to suggesting "npm install". The issue is [this line](https://github.com/gulpjs/gulp-cli/blob/master/index.js#L172).

### Please give us a sample of your gulpfile
N/A

### Terminal output / screenshots

### Please provide the following information:
N/A - This happens in any environment.

### Additional information
N/A

Start at index.js around line 172, where gulp-cli suggests npm install when node_modules is missing. Check the package-manager lockfile condition described in the issue and verify that a project with pnpm-lock.yaml receives the pnpm install suggestion while the existing fallback remains unchanged.

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