The patch below changes the store so that it flushes what it reads from
the substituter’s stderr when it encounters either a newline or a
carriage return.

The intent is to allow substituters to provide some kind of progress
report, where the status is updated by means of a ‘\r’.

As a side effect, this patch removes the “substituter-name: ” prefix on
the lines that are printed.  It can be reinstated if deemed appropriate.

WDYT?

Thanks,
Ludo’.