I believe the issue is due to spawn-cmd which multidep uses internally. CI against Node 15 passed when using [15.11.0](https://github.com/broccolijs/broccoli-plugin/actions/runs/644033024) and fails against [15.14.0](https://github.com/broccolijs/broccoli-plugin/runs/2494354003?check_suite_focus=true#step:3:7).
