Package updates, support for global json file in a subdirectory, installer scripts updates

This release includes the following PRs:

• Adding support for the global-json-file input: #276
  Example of usage:

  - uses: actions/setup-dotnet@v2
    with:
      global-json-file: csharp/global.json
  - run: dotnet build <my project>
    working-directory: csharp

• Update @zeit/ncc to @vercel/ncc: #290

• Update vulnerable packages: #289

• Improve documentation and update installer scripts to the latest version: #278