remove the library path before we exec

In a fix to #148, I added unsetting these env vars into the Boot2Docker.app - but then reading #606 made me realise it should really be done in b2d-cli itself.

so 

Fixes #606 - maybe

This needs testing on a setup that has these env vars tho, as mine works all the time - i presume you need more dev tols installed locally?
ah, poop i assumed it was populated. I don't know that i'm willing to risk emptying it - in case vbox uses the path or something to then do stuff ;(

but ya - I'll re-do using Setenv()