fix: propagate PATH to ovpn in plib

OpenVPN does not inherit its parent process' environment variables by default, but we still want to propagate $PATH when invoking it. This is fixed by using --setenv PATH <path>

Additionally, Some systems such as NixOS don't store any binary in /bin, /sbin, /usr/bin or /usr/sbin so setting the PATH variable in ovpn-client.py breaks the script.