The existing code for setting up the environment assumes that
the directory layout containing rust and cargo conforms to the
one used by rust-installer's tarballs. This makes overriding
the system root awkward for simple cases where I want to test
my own build directly from the rust build directory. This
patch just adds a second path to PATH and LD_LIBRARY_PATH
to accomodate both disk layouts.

Conflicts:
	python/servo/command_base.py