refactor: use getExecOutput and simplify control flow

Replace the hand-rolled `exec` + `listeners.stdout/stderr` buffer
capture with `getExecOutput`, which already returns
`{ exitCode, stdout, stderr }` and still streams live output into the
group. Drop the `groupOpen` boolean (`endGroup()` is idempotent — just
writes `::endgroup::`, repeated calls are harmless), remove the
`finally` block, and flatten the success path with an early `continue`.

Author: fengmk2