west: runners: jlink: Avoid running logic when another runner is used

The logic to detect the default JLink location was placed at the top of
the script, executing whenever the script was imported. The west
extension command framework loads all runners when initializing, and so
this logic was being executed even when using another runner.

Move the logic to a function that is only executed when the JLink runner
is selected, to avoid executing it at all times.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>