scripts: west_commands: runners: nrfutil: Only flash with J-Links

The nrfutil runner calls "nrfutil --json device list" which outputs
information about all connected serial ports. The list includes not only
actual boards but also any ttyACM instance. If the ttyACM instance does
not have serial number, then the nrfutil runner will fail on matching
serial number regexp on NoneType.

Fix the issue by limiting nrfutil runner board output to only devices
that have trait jlink set.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>