ensure: guarantee execution of a step

Any step can have ensure tacked onto it, whose value is a second step to execute regardless of the result of the parent step.

The step to execute. Regardless of whether the parent step succeeds, fails, or errors, this step will be executed. The step will also be executed if the build was aborted, and its parent step was interrupted.

If the parent step succeeds and the ensured step fails, the parent step is considered to have failed.

The ensured step executes after any on_success or on_failure hooks.

For example, the following build plan acquires a lock, and then ensures that the lock is released.

- put: some-lock
  params: acquire: true
- task: integration
  file: foo/integration.yml
    put: some-lock
    params: release: some-lock