Changing NBI Pebble Service command to enter the startup directory first

In rockcraft.yaml WORKDIR is not supported, so it needs to be specified in Pebble service definition by using working-dir parameter.
However, working-dir is not supported in Juju 2.9.x. So, we are entering to the startup directory first as a workaround.

Change-Id: Ia441b5445822f0833f659f964fc1f2216586cdc5
Signed-off-by: gatici <gulsum.atici@canonical.com>