Everything on the same LAN should generally be treated as "compromised/not compromised" together. There's rarely just a compromise of one machine in the same way there's never just one cockroach.

I'm not sure whether distcc affects reproducible builds?

You could, in any case, have tighter controls on the release builds, which would be done on a CI machine before signing.

(Back when I used distcc we didn't distribute across the dev machines, we had an entire build farm of two racks of 2U servers!)