Use std::variant to enforce BuildResult invariants

[Ericson2314]

Motivation

There is now a clean separation between successful and failing build results.

Context

I thought about making a JSON format for it after seeing what @RossComputerGuy got from detsys in #14031, and I wanted to do this first to better understand the data structure.

(I think the JSON format should also cleanly separate the success and failure cases. Among other benefits, such a format that means that even if there is an unknown status code, success and failure could still be told apart.)

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[edolstra]

This is a fairly large amount of code churn (a 1807-line diff) while it's not entirely clear to me what "a clean separation between successful and failing build results" gains us... Can you expand on that a bit?

[Ericson2314]

Motivations include:

• Make the various doneSuccess and doneFailure functions more correct in terms of the statuses they accept
• Allow for a JSON format which distinguishes success/failure separate from the status code, so new status codes that are not understood by the client don't mess up telling apart successes and failures. That makes things a bit more extensible.

[Ericson2314]

Addressed the review items above, and created the tryGet* helper functions as discussed in the meeting.