Use build-specific settings for effect across windows and unix

[dmikusa]

Summary

• On Windows, we use the standard os/exec functionality. As Windows doesn't really have the concept of a PTY.
• On Unix, we use a PTY. This allows commands started through effect to appear like a user is actually running them. In many cases, this doesn't matter, but some commands will change their output if there is a user or pipe. The PTY fakes the spawned process into thinking it's a user. This will generally cause the spawned process to emite human-friendly output.
• This change is due to the fact that recent Windows builds of paketo-buildpacks/procfile have been failing. This traces back to pty.start() creack/pty#50. It's not clear why this is happening now, since the issue is a few years old. With recent changes in libpak, we're now affected by this.
• Initial tests with paketo-buildpacks/procfile show that making the PTY stuff conditional on the OS, allows this to work as expected.

Uses go conditional builds as described here.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).