Fix(): certificate path should not be relative to current directory

[GuillaumeOREVE]

Whenever I launch the simulator from a folder that is not strictly the root of the simulator project, I get an issue at the end of a charging session, when I try to stop it (and only then):

node:fs:563
  return binding.open(
                 ^

Error: ENOENT: no such file or directory, open '/Users/userName/my-current-dir/cert/vcp.pub'
...

This proposition of changes fixes the issue.

The alternative would be to instruct clearly on the README to only launch the simulator from its root folder (but I find it not convenient because I use it in a bash script for easing the process of launching it).