Yeah. It’s only a problem if you’re managing sites that you don’t log in to. For sites that I have an API key for, I can pull /admin/dashboard/problems.json and see if it has mail-receiver in it.
I presume that this won’t be a real problem until the day in the Mysterious Future that support for api key in the URL goes away.
There’s little downside in rebuilding the mail-receiver an extra time or too. I probably shouldn’t even try. 
Thanks for your help!
Edit: here’s the answer I was looking for! Update mail-receiver to the release version