You can now use setup-node action to cache dependencies for projects with monorepo and pnpm package manager. Use the optional cache-dependency-path field to specify the path to dependency file(s).

- uses: actions/checkout@v2
- uses: actions/setup-node@v2
    node-version: 14
    cache: npm
    cache-dependency-path: 'sub-project/package-lock.json'

For questions, visit the setup-node repository and GitHub Actions community.