build_usd.py doesn't pass dependencies to each other

build_usd.py doesn't appear to pass the path to dependencies to other dependencies.

For example, in the link above, Alembic doesn't get passed in Imath_ROOT which means that it tries to find it in a variety of places, including homebrew installs which often breaks the build.

I typically don't use the build_usd.py to do our builds, so I hadn't noticed this before but I just tried it and hit the above. Hopefully an easy enough fix to do, but I think its important to forward dependencies forward through the tree to avoid CMake picking things up "helpfully" from unexpected places.