Multi-worker-docker-container: disable log buffering (#16919)

Background: we have a `matrixdotorg/synapse-workers` docker image, which
is intended for running multiple workers within the same container. That
image includes a `prefix-log` script which, for each line printed to
stdout or stderr by one of the processes, prepends the name of the
process.

This commit disables buffering in that script, so that lines are logged
quickly after they are printed. This makes it much easier to understand
the output, since they then come out in a natural order.