scripts: runners: nrfjprog: restrict when we guess snr

Right now, the nrfjprog runner will prompt the user for which board to
use if there are multiple possibilities and the --snr command line was
not given to specify one ahead of time.

Tweak this so it only happens if standard input is connected to a
terminal. This should avoid stalling the process on board farms when
this runner is used in automation.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>