scripts: runners: defer ensure_snr() to run in nrfjprog

The create() classmethod should not be doing any I/O -- its only job
is to create the ZephyrBinaryRunner instance. It's currently trying to
figure out what the serial number of the board is, though. Let's defer
that work to do_run(), so it gets handled by run_common's exception
handler.

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