sg: stop buffering command output until newlines

Created by: mrnugget

This fixes the first part of https://github.com/sourcegraph/sourcegraph/issues/30529, that it was impossible to do for { printf("a") } because sg was waiting for newline characters.

Caveat here still applies:

https://sourcegraph.com/github.com/sourcegraph/sourcegraph@af21a04b1682d33fea3e69b55df93a822649ccf5/-/blob/dev/sg/internal/run/logger.go?L23-28