BuildResult: Add a "Cancelled" status

[edolstra]

Motivation

This denotes the result of a build that didn't succeed or fail, but was cancelled because some other goal failed and --keep-going was not enabled.

Cherry-picked from DeterminateSystems#217, where I'm trying to make nix flake check output more informative by showing which flake outputs failed, succeeded or were cancelled because of another failure, for example:

Context

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[Ericson2314]

This is good, but it poses a back-compat problem for older protocols.

I think what we should do is get rid the numbers in the definition of the enum, and instead have serializers for std::variant<BuildResult::Failure, BuildResult::Success>. Then if the protocol is too old, we can map the status to something else.

(We might also switch the protocol to strings while we are at it.)

[Ericson2314]

Closing in favor of #15090 which does do the protocol compat correctly