Cleanup on Abort

The following will perform the cleanup task only if the build is aborted while the unit task was running:

- get: foo
- task: unit
  file: foo/unit.yml
    task: cleanup
    file: foo/cleanup.yml on_abort step hook

Any step can have on_abort tacked onto it, whose value is a second step to execute only if the parent step is aborted, via clicking the X in the web UI or by running fly abort-build.

on_abort: step

The step to execute when the parent step aborts. If the attached step succeeds, the entire step is still aborted.