This is a followup to #2892. Since we now attempt to lock the PID file before
starting plugins we need to make sure that we actually use a unique lightning
directory for anything that attempts to call `--help`. If not we may be
conflicting with a `lightningd` that is running against that directory.
Notice that this still means that we will be unable to call `--help` on
`lightningd` if we have a running instance, but isolation in this case is
good, otherwise we'd be reading the default config anyway.
Signed-off-by: Christian Decker <decker.christian@gmail.com>