Multiple commands support on Windows

[tenevdev]

Related to https://github.com/buildkite/agent/issues/1538 but aimed at coverage and consistency with other platforms (or clear documentation about it not being supported).

On Windows, executing the step below only shows the output with the version once, it completes without executing the second command from the list.

    commands:
      - buildkite-agent --version
      - buildkite-agent --version

During execution, the first command exits with no errors so the expected behaviour is to see the second command run.

More broadly, some types of command item such as an executable, may cause the generated wrapper script for Windows jobs to exit in a non-intuitive way due to the semantics of the batch script. Please, make this more consistent with the intuitive behaviour on other platforms.