scripts: runners: fix blackmagicprobe SIGINT behavior

The blackmagicprobe runner's Python process fails to ignore SIGINT
when it runs GDB from the debug and flash callbacks, which is wrong.

The other runners tend to use run_server_and_client() to properly
handle this, since they start a GDB server and connect to it with a
client. The BMP USB device presents itself as a serial device which
speaks the GDB serial protocol instead, so there's no server/client,
and thus no call to run_server_and_client().

The problem is that blackmagicprobe essentially uses
subprocess.check_call() to start GDB directly, without ignoring SIGINT
in the python process. Easy fix.

Fixes: #21139

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