Fix some trapped output

[alexarchambault]

Consider this change with caution. I'm opening this more as an issue than as a PR (although if you think the change is fine, let's merge it).

When printing things to stderr right after this line, without this change, the printed stuff gets trapped. Diving into this with Claude AI, I ended up finding that this change fixes that problem. It also doesn't break the CI.

I don't understand much about all the proxy stream / prompt logger and all logic, so I'm not sure what other implications this change could have.

[alexarchambault]

This has been a big problem for me when debugging things in Mill, as things I print with pprint end up trapped.

[lihaoyi]

@alexarchambault could you dig into the code to see where this variable gets used later on? Should try to figure out what it's doing and propose a mechanism for the bug and fix rather than just changing it blindly.