Passing `--on-backwards-incompatibility=expunge` does not work as intended

[benh]

Describe the bug
From Discord, "I'm trying to run rbt dev run with the --on-backwards-incompatibility=expunge flag but it doesn't seem to be working". See this thread.

Impact
How severely does this bug impact your use of Reboot?

• It's not having any negative impact
• It's annoying
• It's a serious problem
• It makes it impossible for me to use Reboot

Additional context
Running a Node app via yarn, see thread above.