Add warning about using --out in runners/mine.py

Fixes #18390
Welp, looks like @thatch45 already merged this with changes, but didn't "omit" the warning. Should I make another PR or leave it be?

I remember there was a reason @basepi and I thought we shouldn't use the `warn_until` and mentioning the version in the comment (so we'd find it), but now we can't remember why.