pressure-vessel: Make graphics drivers outside $HOME available, if used

Since !173 (merged), we have been able to create symbolic links to graphics drivers anywhere on the filesystem, but that didn't mean the graphics driver would necessarily be usable, because the directory containing it would not necessarily be visible in the container. Add these directories to the list of directories to be "exported".

Manual test (Debian on on Intel hardware, adjust as required for others):

• Modify /usr/share/vulkan/icd.d/intel_icd.i686.json to use /opt/moved-from-usr/lib/i386-linux-gnu/libvulkan_intel.so
• Move the real i386 libvulkan_intel.so to that location
• ./run --verbose -- steam-runtime-system-info 2>&1 | tee container.log

Resolves: #29 (closed)
Resolves: https://github.com/ValveSoftware/steam-runtime/issues/313

/cc @denittis