Experiment: override zero exit code on cancel

[DrJosh9000]

Description

Fixes #1489

Changes

• Add a new experiment, override-zero-exit-on-cancel
• If the job is cancelled, and the exit status of the process is 0, override it to be 1 instead.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[moskyb]

🎉🪟