Always add SIMBODY_VIS..._INSTALL_DIR to search path.

I ran into a scenario where I had set SimTK_INSTALL_DIR, but there wasn't any
simbody there.

Previously, SIMBODY_VISUALIZER_INSTALL_DIR was only added to the search path if
the env vars weren't set. But what if they are set but just set incorrectly?
The SIMBODY_VISUALIZER_INSTALL_DIR should always be added to the search path.