Cleanup with defer

defer lists tasks that run after a task's body finishes, whether it succeeded or failed. Use it for cleanup that must not be skipped.

## cleanup

```bash
docker compose down
```

## integration

```meta
defer = ["cleanup"]
```

```bash
docker compose up -d
./run-integration-tests.sh
```

• Deferred tasks run in the order they are listed, and each may be called with arguments: defer = ["notify status=done"].
• They run only if the task actually started: a task skipped by if or because it is up to date does not run its defer.
• They also run when the body failed, timed out, or the run was aborted by another failing task.
• With retries, deferred tasks run once, after the last attempt.
• The task's result is the body's result. If the body succeeded but a deferred task fails, the task fails with the deferred task's exit code. If the body failed, its error is kept and a failing deferred task is only reported.
• Deferred tasks see the same loop variables and dependency outputs as the task, and in a loop they run after every iteration.
• A deferred task must not have depends of its own.